So I get the Ibanez BTB1206. Awesome instrument. I bring it home, and when I plug it in and turn up the treble I get an oscillating squeal. I try it in two separate amps, and get the problem in both. I try other instruments, including ones with similar Bartolini pup/preamp configurations, at similar volumes and get no such noise. Must be the BTB1206, right? The shop called me just now: They cannot duplicate the problem. I will go there this weekend and try to duplicate it there. In the meantime, I will try my best to duplicate it at my home studio with other basses.
Also, if you plugged both amps into the same plug, try moving the amp so you can try another outlet. You could have a grounding problem.
Turns out it's a 'feature' lol Seriously. The 18v system is more sensitive than others. When I crank up the Conklin, I get the same result. It just occurs at higher volume so I had never had occasion to notice. So the BTB1206 is home. Boy does this thing play nicely!! It came with Elixir Polyweb strings. Since my other basses have very lively tone, I will leave the Polywebs with their more muted tone on for awhile.