Perhaps you can ask John (Silvertones) for the demo-files he used to make his choise.
Here's a short demo I made with the VoiceLive2 (same harmony-engine as the GXT).
This demo is made using a Shure SM58 and an Ovation CC48, both plugged into the VoiceLive 2 and the output was recorded live as-is into Cubase. No further editing was done: the sound you hear comes straight from the VoiceLive 2.
The first bit is The Boxer by Simon and Garfunkel. I used a factory preset (3th above) and tweaked it a little bit to better suit my taste. The first part is only lead vox with reverb. The second verse ads the harmony (foot-switch).
The second bit is Teach your children by Crosby, Stills, Nash & Young. I used a preset with a small tweak. You'll hear 2 3ths and 2 5ths, panned in stereo.
The guitar-sound was enhanced with the VL2 built-in guitar effects.