What is the lowest voltage acceptable ?

Furman should do just fine. I use older PS Audio regenerator that does voltage regulation too, in addition to other things. They are quite expensive even used and don't always work with powerful power amps, it depends on particular model.

If you're worried about it come and get my TrippLite LC-1800.

I’m using this at present: CyberPower CP1500PFCLCD PFC Sinewave UPS System, 1500VA/1000W, 12 Outlets, AVR, Mini Tower,Black https://a.co/d/08AKYmj9

I’d say 115V should be the lowest acceptable voltage; as others have mentioned, standard US wall outlet voltage is now 120 VAC (half of 240 VAC coming into the breaker panel).

BGE got into a decades-long argument here in Maryland with PEPCO over who was responsible for maintaining/upgrading underground wiring that was going on 40+ years old and, for a long time, we experienced frequent brownouts, blackouts, but only when it got very cold, or very hot, or very wet or, very dry or windy (really). Sometimes we’d lose power for DAYS during ice storms or one particular derecho (wind) storm in JULY about 15 years ago.

I’ve since bought military surplus backup generators. 

We also got power OVERvoltages - as high as 135 VAC - which eventually took out several appliances, a TV and a breaker panel we had to replace; insurance covered, I never found out whether the insurance co was able to subrogate and get reimbursed by BGE for the cost.

On a lark, after the underground wires were finally replaced, I measured power levels day to day. They varied by about 5% around 120 VAC, but mostly stayed at/around 123 VAC with occasional peaks to 125, lows to 118.

These days I use the Cyberpower “conditioner”/UPS to maintain an even 118 VAC as I use some older equipment designed to run on 115 VAC; set any lower and the UPS sets off alarms.

220-221 ... whatever it takes ...

I don’t see an answer to your question so far. What the utility is required to provide and what will damage your equipment are two different things. And I don’t really know the answer. Probably the answer is well below any voltage that you will see. You didn’t say what type of gear you have. But solid state gear will probably work fine and eventually trip off. Most ss gear will rectify your AC to DC, and regulate it to a much lower voltage. If the regulator is good, it will never know the voltage is low. Tube gear usually is ok with lower voltages but not always. If you have a good tube amp I’d ask the designer/builder.

You’ll more likely hear poor performance than damage your gear. These are my educated guesses though. I don’t know all the architecture and design features of your gear.

You might find support equipment that runs off of 120VAC to not operate correctly, perhaps a remote volume control or a display system.

What does damage equipment is high current. power = current x voltage So a piece of equipment, such as an inductive motor, that needs constant power, will pull more current at a lower voltage. Refrigerators are the classic home load that can be damaged during a brownout. Still, 109 volts should be fine. Air conditioner motors can fail too. The theme here is motors that pull a significant load. If you ever have a brownout--you can see the lights dimming, run, don’t walk to unplug your refrigerator and large loads.

Turntables have a small motor. Unless someone know knows more about turntables speaks up knowledgably, you might want to not use your turntable at low voltages. but I’d give it 95%+ chance it will be fine at 109V.

If you do want to buy a power supply to fix the problem, PA Audio regnerative power plants are probably the best. I set the output voltage on mine and it controls it +/- 0.1 volts. The specs say it can operate down to 90 volts. Note that this is low enough that you could use it to run 120V equipment in Japan (100 V standard).

Jerry