Step down transformers detrimental to SQ?

Yeah, just get acupwr and you should be fine. I heard good things about them, I would not save a few dollars on this device.
As for sound quality..shouldn't make things better, I'd say. You will want a very good low gauge power cord for amp, anyway.

I heard some crazy Japanese audiophiles do double conversion, up and down, and claim it improves the sound. I should try it one day. But before that I should try to run my Nak deck on 220V - it has custom dual voltage transformer.