You can purchase a transformer here https://airlinktransformers.com/category/japan-uk-voltage-converters and it will work absolutely fine. The frequency is irrelevant for an amplifier.
If it doesn't ever happen when there are no lights on in the room it could be related to that. I had a TV that switched channels occasionally whenever a CFL light was switched on.
Even budget amps these days have flat responses and inaudible levels of distortion if used under the maximum power. The McIntosh isn't going to sound better.
Valves are something we used to use before the transistor was invented. Using them now is like going back to driving a horse and cart and claiming it's better.