pfsmith0

Here's an option to try - use the headphone output with a splitter/adaptor to bring out the L/R signals into different plugs. The headphone output has a gain control that will raise the signal to beyond the 1V required by the Art.

If you need a mic level signal coming out of your HD500 then you should set the mic trim to mid gain. According to the data here, the mic trim has gain anywhere from -20.5dB to +23.7dB when the output is taken from the XLR connectors. Setting the trim to the middle will get you close to 0dB. Mic level coming in and mic level going out.

The KK Trinity Solo Mini uses a stereo TRSS 1/4" jack so you'll need an adapter to split the stereo signal into 2 unbalanced mono signals per space's comments above. The transducers will need to go into the guitar input using the highest input-Z setting on the HD500 (3Meg?). The mic will either go into the Aux input (if its signal is high enough, but I doubt it) or the XLR mic input via XLR-1/4" adaptor. Then adjust the mic level input to get the balance you want. You can also pan the Guitar/Mic inputs L/R to keep their entire signals chains and FX separate and rout them to the L/R stereo outputs.  The signal chain would look like this:
 
Trinity pickup -> guitar stereo out -> adapter to 2 mono signals (prob 1/4" & XLR) -> 2 HD500 inputs (prob Guitar & Mic) -> 2 separate A/B signal chains -> stereo output

I haven't tested the new model paks but so far the flattest amp I've found is the Class A-15 with these settings:
Bass = 25%
Cut = 70%
Treble = 18%
Pres = 40%
Hum (DEP) = 0%
The rest of the DEP parameters can be kept at their default settings as they effect dynamics more than frequency response.
 
As far as I can tell there are no cabs with flat frequency response, but arguably the closest seem to be the 1x12 Celest 12-H and the 4x12 Tweed B-Man.
 
The 121 Ribbon mic is the flattest of the mics.

Yes. You have two signal paths available to you, A & B. Connect them to Input 1 and Input 2, respectively. Set Input 1 to Variax and Input 2 to Guitar and you're all set with two independent signal paths. I think the built-in tuner only works on Input 1, though, so you may want to swap the Variax & Guitar to Inputs 2 & 1, respectively.

I measured the gains of the various inputs and outputs and recorded the results here: http://line6.com/support/topic/848-gain-line-up-measurements/?hl=pfsmith0&do=findComment&comment=4669
 
The Aux input should have the same gain as Guitar Normal. Guitar PAD adds about 6dB of loss at the Guitar In jack. The other differences between Guitar In and Aux is 1) the tuner is only available at Guitar In, and 2) the input impedance of Guitar In is programmable while Aux is 1M ohm. I've seen no other differences between these two.
 
I assume you plug the guitar into Guitar In and then Aux without changing anything else and you're noticing a difference, right? Ideas are:
 
1) make sure the Guitar In switch is set to Normal
2) make sure the Input Impedance is set to global 1M
 
If you still hear a difference then perhaps the unit is defective. I can't think of anything else to do.

I believe the foot switches aren't ON/OFF so much as they are TOGGLE. So if you start with 1 FX ON and the other OFF you can use one footswitch to toggle them both from ON/OFF to OFF/ON. So for you, use two FX blocks, each containing a delay set with a different delay amount (quarter note and dotted-quarter, for example), and set one ON and the other OFF, each controlled by the same footswitch, and it should work. However, if you also need the OFF/OFF state then you may need to use a 2nd dedicated footswitch to turn just ONE of them ON/OFF. But once you get them to be ON/OFF or OFF/ON, then the toggle idea should work.

Bad idea to hook up more than one input cable at a time. The Rokit5 manual also says this at http://www.krksys.com/manuals/rokit/RokitG3-Manual.pdf.

Awe shucks. Someone else got the "best answer" even though I identified the problem. I was this >< close to getting it!

IFRC, there were only a few a ribbon cables but they all disconnect easily. There was a little finagling required but nothing too undaunting. The buttons have a snug fit but can unexpectedly pop off when you, um, don't expect it. The menu control buttons fall out - nothing holds them to the pcb. Be sure you find ALL the screws before you pull too hard. There are screws on the back panel as well as the bottom panel. The first time I did it the knobs went flying off when I pulled the PCB loose, so 1) take them off first, or 2) get a 2nd pair of eyes to watch for flying parts, or 3) don't do this over carpeting. Good luck. If you've tinkered much with electronics you shouldn't have too much trouble. If you HAVEN'T tinkered much then this unit isn't unusually difficult. It may be a good place to learn, albeit it's somewhat expensive to replace. You don't really have a choice, right? You've got to fix the power plug. As I said before, I'd go ahead and replace the USB connector as well while you're in there.