high voltage's affect on amplifier performance

Instead of a Variac, which you will constantly have to adjust manually, why not try a voltage stabilizer, or isolation transformer?
Maybe something like this would help.

Cheers,
John

I doubt that frequency response would be affected. Do you have any measurements to support your idea? Since rail voltages are usually not regulated your amp will have a little more power output. I don't think that 124 volts would measurably degrade reliability but if it gets much higher I would start to worry.

Again I ask...what voltage spec is the power company obligated to meet? I know we have electricians on this site who can provide this info.

Yes, I too don't see how frequency response would be affected. I would think that power output, dynamic range, and linearity/distortion would be subtly improved on musical peaks, at the expense of slightly increased stress on tubes, capacitors, and other components (probably to an insignificant degree, as Eldartford indicated). If the design incorporates regulated filament supplies, that would minimize or eliminate possible reduction of tube life.

Re voltage stabilizers (as opposed to voltage regenerators such as the expensive PS Audio unit), the ones that I am familiar with use a servo motor to actuate a variac-type device. That means considerable mechanical noise will occur periodically (whenever line voltage or load current change significantly), so you would not want to have one anywhere near your listening area. Here is an inexpensive example:

http://www.voltageconverters.com/itemdesc.asp?ic=SVC2000

Regards,
-- Al