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 to choose from.
• 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 the 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 whose 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.