HAHAHA - I know the feeling.  Last night I was contemplating using a 4 cable method to run my X3 Live in the effects loop of the HD500.  I laughed as I looked the other way and saw my XT Live sitting around as well.  I could then attach it to the effects loop of the X3.  And I wonder if I could use the 500 to midi control the X3 and/or XT.  A 3 processor, 12 cable method...ridiculous.

I do intend to do a 4CM with the X3 in the HD500's effects loop, but only to easily A/B the sounds between the two.  I intend to record some comparison clips once I'm confident I've dialed in the HD500.  As far as using it as an actual setup, there's no way I would do that.  You are just inviting disaster.  If the output mode on one of the units got changed, or you accidentally put a cabinet simulation in the chain, or whatever, you'd be screwed.  You'd have to frantically search TWO units to find the kink in the chain.  Also, there's a greater possibility of something getting unplugged or having one bad cable, or having a loose connection...there's just so many things that could go wrong in a live setting.  Plus, it might be frustrating to tweak two units at the same time.  You would be editing the parametric EQ on the HD then start turning its bass/mid/treb knobs without hearing any difference in output and be like WTF, then realize you have to turn those knobs on the other unit.

Using the units in the same chain is indeed possible and would probably work well, but it's definitely a more fragile and frustrating setup.  I'm not sure I'd feel confident relying on such in a live setting, but to each his own.

I agree that Line 6 should have put more thought into these issues.  How f'ing difficult is it to put the same sims from older units in your new unit, especially when they aren't even sims at all but just an EQ!??!?!?!!?  Why put an XLR input on the unit if you don't have at least one pre-amp that is tailored to instruments (vocals/acoustic) you'd connect to it?