I, too, am having this problem on the GO with my Epi LP and my PRS SEE C24.
Since I use my guitar volume control a lot, this may be a deal breaker for me unless I can find a work around or a fix/patch for this.
I see the manual's 4-cable method and it mentions being able to switch between the amp's preamp and the GO's preamp models.
What if you just want to use the GO's preamp models? How do I set the GO?