This is a very interesting thread.
Here's my sixpence worth. Like many here, I have been a big exponent of L6 for years and indeed gig every week with the DT-50 and 4x12. I gave up using the HD500 after months and months of messing about trying to make it work for me, I am deliberately leaving out the words - tone and sound.
The issues I had included, but not limited to:-
Pressing the pedal to engage/disengage wah cuts off the sound temporarily and is a bugger to press.
I found it incredibly difficult to balance volume levels across only 4 patches,
Mod effects would also have an effect on perceived volume.
Trying to work out the signal path and how to use the mixer baffles me to this day.
The list of quirks is really too long when all I want to do is play the guitar instead of meddling. I regard the time it takes to set up an HD500 to be extreme and the results are not worth the effort in time. And don't even get me started on using the L6 link which causes incredibly unmanageable volume increases and what's with dual pre amps when running a mono amp? Surely L6 don't really expect me to buy 2 DT amps. And how does that work live sending out stereo studio direct to the PA, it doesn't, mmm. I could go on all day and some would say there are workarounds for all it's faults, who wants that?
I fixed my issues with the HD500 reasonably quickly by buying new gear. I use in front of the DT50, a Crybaby Wah - MXR 10 band EQ
In the loop of the amp i use a Zoom G3X to provide no more than Chorus for one song, Delay, reverb and of course the onboard Volume pedal - Carl Martin Boost with 3 band EQ and level trim just for the solos. Hey presto! instant pro quality rig.
A channel switch to select between Soldano Clean and Soldano Distortion on the amp.
Simple to set up and the important part for me is that the rig is suited to me. I run the amp in full power mode with the master at 2-3 oclock and the channel two volume at around 11 oclock (obviously) dependent on gain setting too which is usually no more that 10 oclock. Sounds great and blows the doors off.
As a side note, I have bought a Zoom G5 pedal on the strength of the quality of the G3X and I set the G5 up just for jamming along with my Sessionette 75 SS amp. Within 10 minutes and I was not far away from my full gig sound and and for jamming, that is near enough. Their software interface is incredible. It's not for me to even state the comparisons just to say it is so much easier to get what I want from it. For the really keen price, I would say check it out and give it a whirl.
All is not lost with the HD500, I use it at home for recording where it truly excels.