Hey Outlaw,
You new Taylors have the ES2 so indeed, no balanced cable there, just a regular T/S guit cable. This "high pitched" sound you're getting seems
very strange, indeed. Are you a solo act or in a full band? I trust you aren't pushing serious decibels, right? If you're like most typical "bar acts" the volume isn't that loud, and along with the fact that a sound plug didn't change anything, I doubt it is any kind of feedback. Are you using a monitor at all? ...if so I trust you're not cranking it, right? Also, you're not using compression into a floor wedge, right?
Also, is the problem the same on both guitars, and at the same chord/note positions?
You also mentioned "adjusting the pickups" ...describe
exactly what you mean by this, will you?
Your various mixers aren't the issue, as you've noticed. I've used all the above and not had issues at all. I'd suggest you setup at home
exactly as you would live but straight into the A&H sans Boss, and all volume/eq same as you would. Position all gear same as you would as proximity and volumes are all inter-related. Get it as close to your live setup as possible at home, and then replicate the issue. Chime back in with what you hear, particularly with what kind of "squeal" that it is you're hearing.
Edward