Any pedals? If so, what type of power adapter are you using with your pedals? Some of the adapters out there are notorious for high-pitched noises especially noticeable with bright amps and/or bright settings.
Quality, custom built equipment for the touring musician is rarely cheap. The adapter is still cheaper than a MIDI controller or modding your amp. They warranty their work and there's no danger of killing the control circuit of the amp should you mis-wire something. My experience is - touring...
I wouldn't worry about a balanced preamp tube for V6. I've never been able to tell a difference in tone in that respect, though some say they can. I wouldn't do a wholesale retube either - try switching individual preamp tubes with a (known) good tube to see if it's tube related. (Probably is...
Tone, portability, and lower stage volume. My 4x12 days are over. It took a little getting used to, but the subsequent gain in overall volume control is wonderful to these tired ears. When mic'ed, that little 10" speaker can sound huge!