using 110v unit in 220v Asia

You can use a stepdown transformer; it won't reduce dynamics if it is big enough that the core won't saturate even on peak demand. Check your amp's power draw and multiply that by three, four or even five to get the kind of VA rating you need. A bonus will be a lower noise floor and a bigger soundstage. If your budget (or your luck) allows, go for a tranny that combines stepdown plus isolation.

Thanks, it's a 300B amp, so I'm guessing it wouldn't draw more than a couple hundred watts.

However, I've heard it said that line isolation may have drawbacks for use in power amps. Is this point debatable?

I use iso trannies on digital front ends. I have tried them on integrated amps and receivers, though. I did not drive the amps hard because the transformers in question were not as big as I wanted (only about the same VA rating as the amp). For that reason perhaps, I did not notice any effect on dynamics. However I did notice a lower noise floor and a much bigger soundstage. So in my book, the effect was all positive.

If you search, you will find members here who use isolation transformers on dedicated AC lines and swear by them to power their entire systems. The key factor appears to be transformer core size. If the core is big enough, it won't saturate and there will be no difference between plugging your amp into the transformer and plugging it directly into the AC line (except for the lowered noise floor and bigger soundstage, of course).

So yes, the point requires some study to get it right. What you want is a straight stepdown transformer without any extra voltage regulation or power treatment, and you want one with a big enough core. A transformer that is too small, or equipped with coils and other current-limiting devices, will probably make your amp sound less good.

If your amp does only draw about 200 watts, try it with a 750VA to 1KVA transformer.

Thanks for the detailed response, I'll give a larger transformer a try.