Starting discussions about using the L3t and the HD500 together. One thing I have found is the Low Cut which is found in cab paramters in the HD500 will be your best friend when creating and editing patches when running into the l3t. That will delete any low end overpowering or rumble and helps the sounds cut through the mix. . The proper way to connect the HD500 for now until the Line 6 link works between the 2 is to connect left or right xlr out on the back of the HD500 to the channel 1 or 2 input of the L3t. You need that preamp section to generate enough signal to get your volume Make sure to set the highs mids and lows to 50% to hear the true sound coming from the pod and turn off the reverb and modulation effects built into the speaker. Also set the master volume of your Pod to whatever volume and leave it there. I set mine at 2 oclock. Then match the volumes in your patches in the pod to a constant level. Once youve done that start playing and watch the LED at the channel one input of the L3t Keep turning that up until it starts changing from green to yellow then leave that it at the volume level. Now you set your actual volume from the Master Power amp volume knob on the rear panel of the l3t. That is the only knob you will want to touch for volume from there out. . If you connect to the main input on the back the line levels will not match and you will get very low volume. When running a variax you will probably want to run in Floor Monitor or mains mode so you get the crisp highs for the acoustic models. If your running straight guitar the you will probably want to choose guitar mode which turns the highs way down (turns down the horn) and changes the frequencies of the 2x10s to act like a 2x12 cabinet.
There are three ways to get your guitar to the house pa mains. 1 use the left or right xlr output that isnt being used to go to the l3t - if you do this make sure your patches are set at 0% pan or middle position (running in mono), 2 - send an XLR out from the L3t using the Mixed signal through output on the rear panel - be advised that if you change the volume at input 1 this will also increase the signal going to the mains, or 3 miking the speaker. #1 is the best option here in my opinion.
Another tip: when using the Line 6 L3t together with the HD500 on your clean sounds go into the Speaker model edit mode and set your low cut at 180 It will remove all low end rumble you may get and give you a nice sparkle clean tone. This also works great on your high gain amp models.