Harald

If you are using Effects Only from your HD then the best option is probably to use Instrument level (-10dB) throughout.  If you also want to use Modelled amps then there is some merit in setting the 1/4" Out switch to 'Line'  (to balance off the modelled and real pre-amps) but lets talk about effects only for now:

There is a primer on 4CM here and you may want to read through this to get your head straight --> http://line6.com/support/docs/DOC-2504

Cabling

Cable 1:  Guitar Out --> HD Guitar In

Cable 2:   HD500 FX Send --> Amp Input

Cable 3:  Amp FX Send --> HD500 FX Return (Left)

Cable 4:   HD500 1/4" Out (Left) --> Amp FX Return

Amp Settings

Master volume(if you have one)  = Low (for now, while you are setting up.  See the note about master volumes below)

Amp Loop Switch  = -10dB (Instrument level)

Amp Loop Turned on !

If the amp loop has a 'mix' control, make sure this is set to 100%

HD500 Settings

Guitar Input Pad = Normal (unless you hear clipping from your guitar due to very powerful pickups)

Master Volume = 100% (See the note about master volumes below)

1/4" Out Switch = 'Amp'

Loop Level Switch = 'Stomp'

System Output = 'Stack Power Amp' (within system settings)

Note on Master Volumes:     on many amps the Master control is implemented after the effects loop.  On some other amplifiers the master control is implemented before the effects loop.  In this case the output volume on your effects unit becomes the master control for the amp and you may need to turn the master control on your amp way high to provide a signal into the effects loop.  Equally, you would need to turn the HD500 Master level down !

Ok.  That covers the cabling and switching.  Now in terms of creating patches ....

1. Start off by creating a completely ‘Blank’ patch with no effects set and no amp model selected.
2. Move the mixer device to the very end of the effects chain by moving all of the effects blocks in front of the amp model.
3. Add an ‘FX Loop’ effect to the effects chain somewhere near the middle.
4. Add a studio EQ just before the FX Block. No EQ changes should be set but the gain should be raised by +6dB (if the guitar input level switch is set to 'normal') or +11.5dB (if the guitar input switch is set to 'Pad'). This compensates for a level drop between the HD Input and the HD FX Loop.
5. The mixer channel A should be set to 0dB -1.5dB and Panned Centre. Mixer channel B should be set to 'Mute'.
6. Any effects that you want ‘in front’ of the Amp can now be inserted into a free block to the left of the ‘FX Loop’ block in the effects chain. Typically these might include Compressor/Overdrive/Distortion and Wah.
7. Any effects that you want in the ‘Loop’ of the Amp can now be inserted into a free block to the right of the ‘FX Loop’ block in the effects chain. Typically these might include Delay/Modulation and Reverb.
8. Try and ensure that the ‘Output’ or ‘Level’ for each individual effect does not make the overall volume level louder or quieter.  This will avoid level problems with the patch overall.
9. Set ‘Input 2 Source’ to ‘Variax’ even if you do not have one.  This will reduce background noise on higher gain patches and will help keep the input gain at the right levels

That is it.  Bear in mind that the level settings are a guideline and can vary a bit from amp to amp.   I wrote a more comprehensive guide specifically for the Marshall JVM that has more info and tips that may be relevant.  This is posted here --> http://jvmforum.com/phpBB3/viewtopic.php?f=9&t=5333 and may be worth reading also.  Other guides are posted on this board too.

Cheers.

JIM

Edit 21 Nov 2011 : after originally posting this I created a document describing this setup and giving an alternative one that can be used if your modelled amps are much quieter than your real pre-amp.  This can be found here -->http://line6.com/support/docs/DOC-2522

I have both an HD500 and an XTL floorboard.  Right now I'm only using the HD500 and the XTL is collecting dust.  I have worked up a couple of two-amp patches with a clean amp and a dirty amp for the HD500.  Of course, this eats up DSP power like crazy and I don't have all the effects that I want or need.  So, I'm wondering about using some of the effects in the XTL.  If I wanted to do this, use what DSP room I have on the HD500 and then add some effects from the XTL, how would I need to connect them?

I tried the 4 cable method with my HD 500 and DT50. I used the sudio eq and added the boost. Double checked the cables etc .. I had to crank the amp way way up just to be able even here the guitar. I read that some setups may require the amp to be turned up, but the volume drop was so huge, I can't see anyone ever being able to play out like that. The amp just wouldn't get loud at all. This was with the HD500 master on full.  I can only assume, I did something wrong ..

Any guesses as to what look for ? I like the DT 50  models, and being able to kick on/off  some effects  is about all I really need at this time so the 4 cable is very intriguing to me, if I can workout my level drop issue.

Jim

It doesn't say on rear or amp or in the Marshall manual what the difference is between having the send level depressed or out. I have tried both in and out while playing and there doesn't appear to be an audible change between the 2 options.

The good news is after a complete recheck through your 4cm package Jim I finally have a decent few tones. They seem good at garage volume level, lets hope that at gig level volume next time it all works

Thanks