I suggest the following to get the results you are looking for...

1. L6 Link to the DT25 from the POD HD500
2. Call up a Blank bank of patches.  Since they are grouped in sets of four, this should work out perfectly for your desired results.
3. Starting with bank # ?? on patch A…How about Bank 1 – patches A thru D?  Good as any if you don’t have any preference?
4. You should see nothing but dots across the signal chain plus one larger black box (where an amp would be), and the Mixer.  Perhaps even the looper depending how you chose your display set up.
5. Put the arrow over the larger black box, and then double tap quickly the Enter button tab on your HD500 unit.
6. For topology 1, choose any Fender full model amp, not just the pre version.
7. Once chosen, Click view to return back.
8. With the new amp still with the arrow over it - double tap quickly the Enter tab.
9. Now you should see two rows of controls.  Both rows with the exact dials as on the DT25 amp.
10. You will be now controlling only Channel 1 which is the only channel you will gain control over the 4 topologies.
11. Adjust the setting on the top row only for now.  This will produce the Topology 1 sound you want.
12. Be sure to save to settings by using the save tab on the unit.
13. Once accomplished, then go to Patch B and repeat steps 5 through 12.  Except this time, choose any of the Marshall (Brit) full amps to create Topology II.  This probably has the widest variety of difference between just the Marshall amps.  Choose the JTM-45 if you want less input gain or J-800 if you want the most gain from the Marshall amps modeled.  Or my personal favorite, the Plexi Lead 100 Brt, for a terrific middle ground between input gain and classic British tonality.  you can always put an overdrive or even a full distortion pedal in your signal chain once you've gotten the topologies you wish to have control over.
14. Using Patch C for topology III repeat steps 5 - 12 except choose the Bogner (Bomber Uber) Full amp.
15. Using Patch D for topology IV repeat steps 5-12 except choose the Engl (Angel F-Ball) Full amp.

  Lastly - you need to do the following to get the utmost from the DT25 amp you have...   

• After tweaking and dialing in the sound you want to come from all four topologies on channel A of the DT25 amp, you then must set Channel B to what you want all the time from that channel.  You can go to a screaming lead sound with tons of gain or to a crystal clean sound with little or no overdriven sound at all.  This is your amp, your sounds. 
• With this set up - you actually have five amps. 
• Two will have the same topology, but you will be making these channel B choices physically on the DT25 amp itself.  Once you set them, they must not get moved or it will change the sound from that channel.  Just double check that channels settings before you begin to play and you'll always have the five amps you've chosen to use.

  To access Channel B you must do the following for all four of your A-D Amp patches.   

1. Highlight the amp one each of your A, B, C, & D patches - one at a time of course.
2. HOLD DOWN the ENTER Tab, and now you will see an "FS#" or perhaps "None"
3. Assign this to FS8 which is the last Footswitch button next to the Pedal.  This is the FS # I always use, your choice is your choice, but I've found that keeping it the same, is much less confusing in a live performance situation trying to remember where the control for something is.  As you add FX, this will become easy to understand why.  My choice is my choice, but you can use any of the footswitches, or even you can use the Toe Switch on the pedal.  I don't do this because I use that pedal for controlling volume most of the time , but these are choices for you to make.  The right choices are the ones you feel comfortable with, not anyone else's choices.
4. Now that you have assign a FS to the amp, you now need to see what happens when you turn it off.

• The amp model shuts off - only the amp - and now you have a volume knob alone which setting can be saved with your patches.  This volume knob controls the volume only of channel B on the DT25.
• Any other FX or pieces you've added to your signal chain will now be heard through that Channel B, unless you choose to assign that same FS switch to turn off or turn on any of the FX in your signal chain that you want to work with DT25 Amp channel B at the exact moment to switch to channel B on the DT25.
• You need to do this for each patch A thru D you create and keep in mind that whatever you choose for channel B, you can always have FX in front of the amp, or behind the amp using nothing more than the L6 Link to its fullest extent.
• For example, you can set up channel B to be very clean.  You then could on one or more patches, have an overdrive or distortion pedal on immediately, or switch on when you want it.  This goes for any effect in your signal chain you allow to work with Channel B.
• It's so flexible, I could write a book that would take up far too much more time than I have to give about al the choices you have in front of you with just these two pieces of Line 6 gear.
• You also could do all these things with the POD HD500 Edit software instead of on the HD500 unit itself and then save it to a Bundle or Set List that you can store and recall anytime you have the unit connected to your computer.  You could start building your own library of patches.

  I hope this small instruction guide helps you discover what your gear is capable of doing.   

Take care and best of luck,
Neal

PS - if you like this guide and it does help you get what you're looking for. I'd appreciate it if you posted how this worked for you.  I can't learn more unless others write back and let me know how my suggestions or instructions worked for them.  Thanks.

Addendum:

I only made the choices I suggested based on the ease of use working with those amps getting a wide variety of tonal options.

I do agree that the Vox 30TB or the Vox 15 are also terrific choices for Topology III if you want less input gain choices and stick within that British invasion scope of sounds.

I am not suggesting that my amp choices for each topology are the only ones.  There are many that use Mode I, and Mode III.  Marshalls pretty much cover Mode II but are not the only choice in this mode.  And Mode IV seems to be only used by the ENGL Fireball model.  I think if Line 6 had more than just the Dual Rect Mesa verson, you would probably see mode IV with other Mesa models.

Okay - here is a complete list of what modes switch when using the default amp models...

Mode I

• Fender (all models except the Tweed Bassman)
• HiWatt (this is surprising to me, but it's what it is.)  I always thought HiWatt was more similar to a Marshall.
• PhD Motorway

  Mode II  

• Supro - Pentode/Class A Output
• Marshall (all models)
• Fender Tweed B-Man (The original Marshall amp. True origin of the Marshall JTM-45 included at the bottom of this post.)
• Ampeg Flip Top Bass amp

  Mode III  

• Vox 15 - Pentode/Class A Output
• Vox 30TB - Pentode/Class A Output
• Gibson 185 - Pentode/Class A Output
• Divided 9/15
• Bogner
• Mesa Dual Rec
• Line 6 Elektrik

  Mode IV  

• Engl Fireball

This is the complete list of every model amp on the latest upgrade 2.02 for the POD HD units.  All topologies listed are the ones that come up before any user adjustments or changes.  All amps are Pentode-Class A/B unless listed otherwise.

I hope this clears up any possible confusion to my amp suggestion in my tutorial.  The amp choices are 100% at the users discretion.

Marshall History - the original JTM-45.... (artical taken from Wikipedia)

Jim Marshall thought he could produce a cheaper alternative to American-made guitar amplifiers, but he had limited experience as an electrical engineer. He enlisted the help of his shop repairman Ken Bran and an EMI technician named Dudley Craven, and between them they decided they most liked the sound of the 4x10" Fender Bassman. They made several prototypes using the Fender Bassman amp as a model. The sixth prototype produced, in Jim's words, the "Marshall Sound".^[3]

The first few production units almost copied the Bassman circuit, with American military surplus 5881 power valves, a relative of the 6L6. Speakers were then rarely able to handle more than 15 watts, which meant that an amplifier approaching 50 watts had to use four speakers. For their Bassman, Fender used four Jensen speakers in the same cabinet as the amplifier, but Marshall chose to separate the amplifier from the speakers, and placed four 12-inch Celestion speakers in a separate closed-back cabinet instead of the four 10-inch Jensens in an open-back combo. Other crucial differences were the use of higher-gain ECC83 valves throughout the preamp, and the introduction of a capacitor/resistor filter after the volume control. These circuit changes gave the amp more gain so that it broke into overdrive sooner on the volume control than the Bassman, and boosted the treble frequencies. This new amplifier, tentatively called the "Mark II", was eventually named the "JTM 45," after Jim and his son Terry Marshall and the max. wattage of the amplifier. Note: the actual wattage of the JTM-45 is 30 watts.^[4]

I seem to be having the same problem as a lot of people on here; I bought the DT25 and the HD500 on the strength of the sound of the DT25 and the idea of being able to control that sound transparently using the HD500. I've read so many posts with varying ideas about how to go about this and nothing I've tried gets the same sound using the L6 link as running straight into the DT25, Why can't Line 6 explain what settings the DT25 uses so that we can recreate those on HD500, my gut feeling is that they haven't because it can''t and that there is something fundamentally different about them.

I bought this amp because it's the best sounding valve amp I've ever played but it's so frustrating not to be able to use it as I envisaged. I'm now looking at using it as a multi FX but then I'm going to have to run an annoying ammount of cables across the stage in order to run some FX in the loop and some in front of the amp.

Willl someone from Line 6 please comment on this issue?

Not 100% same settings on the pots, but it sounds the same:)