Thats a bit pricey
http://www.maplin.co.uk/Module.aspx?...ME R&doy=27m7 I had to replave a variac at work today, yes you could power a multi block from one and vary the voltage.
I'm not sure a SCR device as found in many light dimmers would do the job with amps as the load as the way they chop the incoming mains may come through on the amp as noise
So I would probably go with the variac
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