Here's a link to AC/DC Guitar/Amp tech discussing the Kikusui Power Supply it starts at around: 9.26 into the video. Pretty brief but relevant!

https://youtu.be/j5C7GKGxICg

Furman Voltage regulator are available in 230/240V AC but they are no good for tube amps. They won't harm them but they can't keep the voltage within +/- 2 VAC of the specified 230/240V. They have a +/- 10% so that means if the voltage is within the range of 240V you will still get a varying voltage until the voltage either exceeds 240V by 10V or drops 10V below.  Obviously 240 is not what you are going to get from your socket all the time. I find with tube amps especially if the internal/plate DC voltages are high. My custom buily Barwatt "Super P.A"100W 1968 circuit amp runs at an internal 475 VAC. Even small fluctuations in AC voltage, I'm talking 2 VAC, you can here the difference. The builder set the tubes bias with a outlet voltage of 240VAC. If the voltage is running high, say 245 V the amp sounds brighter, harsher and not so good.

When the voltage drops below 240V to around 238V it sounds excellent, further drop in voltage to around 232 V it again starts to sound bad.

Bands/Musicians like AC/DC and Joe Bonamassa and many more use Kikusui (Japan) voltage regulators. They are quite expensive but they can be used anywhere in the world, you can set the voltage to just about any voltage you desire and it will keep it within +/- 2 VAC of your desired setting( on the 200V setting) and +/- 1 V on the 100V setting. They can store  up to 4 voltage programmes and you can select between 50/60hZ. There is a whole range. I think the smallest  is 500 KVA which I think is enough to power a 100W tube amp.

AC/DC (Angus Young) uses the largest which can power up to 12 x 100W amp heads. His amp tech swears by them. They are essential to maintain a consistent tone worldwide.

There is a Youtube video, a rig rundown either of AC/DC or Angus Young's gear.

I am going to get one when I can afford it as I am getting so frustrated by inconsistent tone from my amps.

I'm in Australia and we have a nominal voltage of 240V at 50 hZ.  I have a plug in voltmeter and it shows that the voltage here is constantly varying. The highest I've seen is 250.8 VAC and the lowest 231 VAV.

I don't bother playing if the meter reads over 245V as the amp just doesn't sound great.  You can actually hear the change in tone when you are playing when the voltage changes by just a few volts.

Furmans' will protect your gear and correct the voltage if it is 10 V either side of 230/240V. I had one and I'm not really sure it did anything to keep the tone consistent. They can be set to 230 or 240V AC.