+1 on importing your file in audacity, it will be easier to sync this way.
- i would suggest you to adjust the output volume on your HD500, using the mixer on board.
-I never tried the GSP, but if you are satisfied with the HD, why change?
-On the HD edit software, you can choose a parameter, under controller, and assign it to EXP-1
You can always check on the line 6 comunity for new tones, or make your own from scratch.
Hope i helped!