Can anyone tell me what an acceptable level of buzzing from a 100 Hz set is? Does a technical definition exist and where in the small print might it be? (background -I have bought a set that buzzes from an Internet based suppiler based on a demo of a non-buzzing set in a local shop). The tolerance levels sound a bit woolly to me...

I doubt there are any hard and fast rules about buzzing noises. It would depend on the frequency of the buzzing etc. I guess, if the noise annoys you, then send it back. I had two sets (Panasonic TX28PB50) which I got refunds on because of an annoying noise.