Hello, my AC15 has started to develop odd low bass overtones in the middle of the neck particularly on the "a" and "d" strings. It also does it on certain note combos. Everything else about the amp sounds great and has since the day I got it about 2 years ago. The overtones aren't overwhelming but very annoying and umpleasant. I record with this amp alot and obvioulsly can't record with it like this. I have changed all preamp tubes and that didn't fix it. I play it daily and get it pushing pretty hard frequently so I guess I should change the power tubes soon, especially since I never have. Any other suggestions? Caps? Thanks!