What is the appropriate capacity of a matching step down transformer for a power amplifier?
Your 230 to 100VAC stepdown transformer should be rated at 400 watts or more. Remember that at normal listening volume levels the power requirements will be much lower than 400 watts. If you use a much larger transformer like 2000 watts it is still safe. However the larger transformer has more iron and hence will waste more power at light load. Say if the transformer magnetising power loss is 2 % then the 400VA transformer wastes 8 watts and the 2000 watt transformer wastes 40 Watts. These losses are relevant if you are playing at low volume 24/7. If you go smaller which I do not endorse then the stepdown transformer may overheat at high audio output power and cause droop in the 100 VAC meaning that the Amp will not produce its rated output power anyway. If you must skimp on the stepdown transformer then thermal protection is compulsory.