Are any of you using a guitar that has a stereo output jack? If so, I could use your help...

Prior to today, I've always used my HD Pro in a pretty standard fashion: guitar cable into the HD's front input, and headphones in the 'phones jack. That's it. No amp.

I recently had a piezo system installed in my Edge Pro 7, and as part of that install my guitar's standard output jack was swapped with a "4-conductor stereo output jack" (tech's words, not mine). Suddenly I found myself in new territory, with no real clue what I needed for cables and whatnot. So, here's what I've done:

1. 1/4" stereo TRS cable (male-to-male) for the guitar

2. Y-cable (1/4" female stereo to dual 1/4" male mono's) to go on one end of said TRS cable

3. Plug Y cable into the HD Pro's left-and-right line level inputs on the back, with -10db selected

4. Configure said inputs for Input Source 1: Guitar+Line 1, Input Source 2: Guitar+Line 2

This setup works, but when I apply any amount of gain I get a steady, low-level hiss that's kinda annoying. I can quash it with a little noise gating, but first I'd like to make sure I've got my basic setup right. The hiss is only present in the configuration I listed above.

Anyone playing a piezo-equipped JP or similar guitar that can give me some tips?