bjnette

You can do both.  Image one;
The Send will send a stereo send if it has a TRS jack split into two TS jack cables. Better than a Y cable as you will have stereo.
 Image One is simply a stereo version of the 4 Cable Method you can call the 6 Cable stereo Method. Congrats!
 Image 2 is standard 4CM with a split into two power amps and could be called more problematic.
Image one will give you the choice of either using the preamp in one or both amps with FX before and after the either both amp preamps or only one and you can use dual or just one side HD preamp models which return to your amp's power amp stages in both amps. 
 
Both ways will need some serious level matching but once you have a setlist and some patches especially for it;
Should be awesome!
Wish me happy 1,000th post! lol

In a nutshell, TRS and XLR are both balanced. XLR brings the level to a mic level signal for a mic pre and TRS would be slightly louder line level which could also go into a line level preamp or line in on a console.
It depends what you are going into which seems to be your monitors by your post.
 
The monitors XLR input will be a lower level ideal coming from a mixer summing alot of channels and ideal for long cable runs, so too the TRS if it has it but most likely unbalance TS jack.
 
The TS jack will be louder and the RCA  probably louder again into your monitors. Won't need to crank the monitors so much.
If the cable runs are short the RCA or the jack will be fine coming from the HD.
If the RCA is a digital input that will be louder again.
 
I think the tone of the s/pdif into my interface sounds better than the XLR outs via a mix board. Have'nt tried it using the TRS outs which will probably be better with less gain needed from a channel input.
Depends on the desk your going into. 

Understood. Put it on the Aux or Mic in or your own DI / Pre to FX return.
 
The p'up signal needs to be pre amplified and both Mic pre and Aux in can but the Aux in will probably be a better impedance match but try each out.
Manual Quote
"21. Aux In - This mono, unbalanced 1/4" input can be used for a second guitar or just about any other instrument. Visit the System & I/O edit pages for Aux In routing options."

It could also be the switch getting some debris in there from the rough use and compressed air will do the trick. The unit is easy to open up and you can check the hot glue connections too.
There are links to be searched for replacement switches if needed.

Insert a hall EQ and lower the tone a lot.
If not dark enough follw with an EQ and cut the high shelf.

I often don't use any gate but some high gain needs to be quiet when not played.
I find the hard gate to give an artifact which can be used for the stutter effect.
The noise gate is much smoother and maintains the tone better to my ears.
Even the stock preset is about right.
To know it hit a note and adfust the threshold. This is what level the gate shuts.
Use the decay to soften the transition. In other words slowly shuts.

The Quick Start Guide has your answers;
http://line6.com/data/6/0a06434d86d0507ef815d3fac/application/pdf/POD%20HD500%20Quick%20Start%20Guide%20-%20English%20(%20Rev%20D%20).pdf
The left rights are standard TS 1/4 "
The FX send returns are TRS if needed.

You could disconnect the HD500 and go into the guitar amp input. That would be true bypass LOLOL
What do you want to do exactly.
 
Try guitar- cable-Amp-Amp send HD500 Effects return-select FX loop and put it where you like after the amp block for starters.
Go HD500 output back into Amp FX return.
 
The 4CM lets you put HD500 stomp FX  before your   Amp's preamp and Rack FX ,in your Amps FX loop.

I use Sonar but am going via a MOTU 2408mkIII.
When you insert an audio track, in the inspector pain click track and on the input you will have a list of available devices,
You can choose left, right or stereo of the USB from the HD.
You will now get left and right selecting stereo. On the same track
 
Or you  can insert two audio tracks and select Left and right input for each.
If no joy check the patch you are using.By adding a stereo FX after a Amp block you should be getting stereo. Also look at the mixer levels and pans.

Have you recorded other instruments in GB? As it might be in arming it.
Check buffer size as well as too little and it won't want to work. Check other setting as well.
Hope your up and running