high voltage's affect on amplifier performance

That is not enough to cause a problem. If you had 135v I would start to worry! (conversely, under 105v I would be MORE concerned than having 124v)

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

Mr Tennis, so has the cost!!

+/- 10% voltage swings from a transformer's voltage rating can cause
transformer hum.

can voltage variation affect the performance of preamps and cd players or dacs, especially if power supplies are not "robust" ?