Ok mixed bag, but I did get a chance to hook up my rig at home, and run some pedals in the FX loops.
First thing, dannnggggg, Blackhawks lost 0-4; glad I was playing guitar that whole time..
Anyway, in short, here's what I got
-DT25 FX loop works just fine while using L6Link from the HD500.
-Placing the Keeley 4-knob compressor in the HD500 effect loop also works nicely, with:
--- noise gate after the FX loop block
--- hd500 drive FX before the FX loop (screamer, tube drive,etc)
-Keeley (bootlegger) overdrive also did well in the FX loop, before the Keeley compressor
Some interesting things occur when you set the Keeley to clamp down the output volume somewhat, while juicing the sustain and input gain. *Almost* allowed me to control the volume jump from the magnetic pickups to the modelled variax. But not quite - though tamed that a good bit - may be time to update the JTV firmware and take a pass at learning Workbench HD..
I tried using HD500 drive effects before and after the Keeley with mixed results. Before actually warms them up nicely, and keeps volume jumps from getting to severe. Allows rolling off the guitar volume to clean up the tone without dropping much volume. Using the HD500 drives/boosts after the fx loop block acted more like a clean boost; I ended up trying different boosts and compressors here, with also mixed results.
Wah through the fx loop into the Keeley was a bit weird, I need to tweak some settings there, try different wah models, and maybe even control the sweep range to keep it from getting too treble or bass intensive.
Chorus pedal in the DT25 effects loop seemed to be the best place overall for it; though it did weird things (not all bad) when I used the looper, and the playback went through the chorus pedal. I also moved the chorus to after the Keeley compressor in the effects loop, and that sounds pretty nice - though it definitely wants to be a post-amp effect rather than pre-amp model.
Last but not least, my M20d touch screen is on the fritz again
I had to reboot it *several times* (more than 10 to 15), load into the "setup / calibration" mode, and do this over and over before it would even properly run the touch screen configuration. I press the button to start the touch screen calibrate, and it flashes the cross mark for less than a second, then snaps back to the main screen, but booting from there still left me with a non-functional touch screen. Then after several reboots it would sometimes display the first (top left) calibrate red cross, but touching and holding didn't function properly, it's supposed to switch to green and say release, and it just stayed red and was stuck - rebooted over and over and over again. It finally, randomly, for no apparent reason, eventually allowed me to perform the screen calibrate. It took almost a half an hour to get from first boot up and attempt, to actual calibration success.
Needless to say, it's going back to Sweetwater tomorrow, to get repaired or replaced.