Another wall voltage question

My wall voltage is 118VAC on a normal day, but when it gets hot outside and every air conditioner in the neighborhood is blasting, it can fall to 108 VAC for much of the day.   My Sachs preamp starts to hum when it falls to 111 VAC, so I've been using a (borrowed) variac for that.  A proper power conditioner/voltage regulator is cost prohibitive given that this cheap fix works for me.  But here's the question:

I've ordered one of the Chinese 10 amp variacs.  I could use it just for the preamp, or I could put it between the wall and my surge protector and run the whole system through it.  The question is whether there's any benefit (sonic or otherwise) to running the whole system, including the ST-120, at a proper 117 VAC rather than the 108 VAC coming out of the wall.  System is VPI Scout --> Bottlehead Eros (PH16x coming soon) --> ST-120 --> Troels Gravesen Fusion speakers, with a Bluesound Node 2i for streaming.  

The wall voltage is 115 VAC now and it sounds amazing.  Just wondering how to optimize for low voltage afternoons.

For symmetry's sake I'd want the whole system running on the same voltage. At 108 VAC, your amp is probably not providing enough voltage to the tubes and their THD may be rising even though you're not hearing artifacts like the buzzing of your pre. Also, the motor on your turntable might slow down with lowered voltage.

Thanks.  That's exactly what I was wondering about. (Interesting that the turntable might run more slowly at low voltage; that would actually be a good thing, since mine runs fast.)

I don't think the turntable would run slower at lower voltage.

Can confirm: Still running fast (34.5 rpm) at 109V.

My turntable is a Music Hall 5.1. I noticed it running a little fast one day while spinning a mono copy of Help. I could tell immediately that Lennon's voice was pitched a little high.
I downloaded an app called RPM on my cell phone and used it to figure out that my table was indeed a touch fast.
(The first several uses are free, then they need $10 to keep using it. I felt it was definitely worth it.)
I bought a Music Hall Cruise control and with the +/- speed correction on the box and the RPM app I got my speed set at 33.34. John's voice sounded just like it did coming out of the speaker on my my Dad's 1964 Ford Galaxy.
You'll have to check to see if this would work on the VPI.

Since my line voltage nears 126 on some days, I use my Variac to bring that down to about 118. Am I to understand a Variac will also boost voltage up if you have low line voltage? I always thought they were a one - way device, acting as a limiter so to speak. I never had to use one to bump voltage up so never thought of them in that way.

I've been playing around with my new fire engine red 20 amp variac, and so far it easily boosts the voltage for the whole system from 110 VAC at the wall to 117VAC.  No audible artefacts, so this looks like a complete — if slightly industrial-looking — solution to my problem.

A friend of mine had a similar problem. It was expensive (like around $250) but he found a sine wave UPS with line regulation. He set it up for the lowest voltage in the worst case which was around 106 volts. Then used a cheap variac just like the one you got to boost it to 120 volts. he never had a problem after that and can even listen to his turntables with the lights go out.

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It's cool to the touch even when it's boosting by 10V.