Voltage conversion of Accuphase A-300

That's a great question, and a great amp!!!

Unfortunately nobody has responded. 

I wonder if I could figure it out if I applied myself. 

I'd contact Accuphase directly.  They should be able to point you the proper licensed service center/tech who can do the change.

The issue is not just the voltage but also the frequency (Hz).

Apparently half of Japan runs on 50Hz and the other half on 60Hz, I stand to be corrected.

It maybe easier to get an audiophile step-up transformer, there are many available because this is a common thing.

Apparently half of Japan runs on 50Hz and the other half on 60Hz, I stand to be corrected.

One side of Japan bought German generators (50 Hz) back when they first electrified, while the other side bought American generators (60 Hz).

As far as the Accuphase, it’s likely its transformer can handle various voltages because it makes sense for manufacturers to standardize SKUs across geographies as much as possible. The conversion could be an easy job - or not.

Accuphase importers take a dim view of gray-market imports. They might charge a fortune to do it, or refuse to help. A good tech will be needed.

If it were necessary to convert frequency as well as voltage, you’d essentially need equipment that converts 230 V 50 Hz to 100 V 50 Hz to DC then back to 100V 60 Hz. Considering the amount of current these amps need to sing, this is most likely not a viable proposition.