I'm read before about it being best to have two seperate isolating transformers, although I had thought the split was usually one for digital components and another for analogue ones?  Were you thinking of your combined DAC/Pre, or is it a question of power amp on it's own whatever?  Anyway, it was partly the fact that at £50 a go I could easily get two of them that in part made the isolation transformer I started this thread about so appealing 

 

Cheers for that    I'll bear it in mind.  I can't really stretch to the 1kV Trichord at the moment though, which it sounds like is the minimum I should consider.