With a 100-watt amp, you are likely only pulling about 150-200 watts out of the wall. This won't draw down the voltage much if at all. Now once you have several amplifiers going full steam, the amperage draw will increase a bit, dropping the voltage down perhaps a few volts. A few volts is of little concern. The voltage out of the wall will fluctuate that much or more over the course of the day.
I have been doing sound for roughly 20 years and haven't come across a band yet, where all the backline instruments couldn't work on a single 20-amp outlet. Most amplifiers only draw between 100 to perhaps 300 watts. A 20-amp circuit has 2400 watts available, and a basic 15-watt circuit has 1900 watts or so that it can handle. So as you can imagine, you would need a band with 5 guitarists, a bassist, and a keyboardist with 4 keyboards to fill up a full circuit. By the time you start getting a full band plugged into a single circuit (speaking only of their instruments), you can see now how even a basic band is of little concern for a single 15-amp circuit. You need to be drawing nearly 90% of the circuit's available wattage before extreme voltage drops can become an issue. For most basic devices such as amplifiers, a few volts, and even 6 volts is not going to hurt anything. I would say that the point of worry comes when you are dropping more than 10 volts over a sustained period of time. I.E. if you hold a HUGE bass note for 10 seconds and the voltage drops 10 volts and doesn't recover, you are coming near the end of the line for that circuit. That is not a practical reality in most music though. Every time you stop playing even for a split second, the current demand stops, and the system will recover from the previous current draw.
Music is extremely dynamic and especially with distorted guitars where the signal is extremely compressed ( because of the clipped signal ), the current demand from the wall from each instrument is meager. 99% of bands are formatted pretty much the same. So a 5 piece band is not working a 15-amp circuit much at all, assuming they are on their own circuit.
If your band's rehearsal space only has a couple of outlets that are tied to the same breaker, you could have issues if you have the band's PA and the backline running full tilt. A basic powered PA speaker has a typical rating for its 1/8th power usage. What this means is that when the speaker is operating at 1/8th of its full potential, it will start to limit and control the speaker's output locking the power consumption down to the speaker's rated current draw, typically around 2.5 to 5 amps. 1 amp at 120 volts is roughly 120 watts. So 5 amps of total current draw (the highest most powered PA speakers are rated for) is only 600 watts. So, when the speaker is screaming bloody murder and every red light it has is on and flashing, it will only be drawing roughly 250-600 watts of power. This means that you can have 2 powered PA speakers running at full potential (full-on clipping) and only just be getting into the worry zone for a 15-amp circuit. Often you can run up to four powered speakers on a single 15-amp circuit before you start running into issues. The reason being is that it is pretty difficult to get all four speakers to start clipping simultaneously; unless you are demanding significantly more than the speakers have to give in the first place.
So to get on point for the OP, I don't think voltage is your issue. I would start by trying new tubes or at least known good tubes. And if the issue persists, then and only then start looking into more serious work. At the age of the amp, I would suspect that a re-cap is probably on the books.
