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I just purchased a new set of NAD components

Thank you for not saying you just purchased a set of NADs. :P

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I can get the buzz to lower by lowering the gains on the back of the amp, though this effects sound quality and level.

Yeah, that'll mess up your gain staging.

The first thing I'd do to check for ground differential would be to bind the grounds... that is, run a wire from a bare metal spot on the pre toa bare metal spot on the amp, this will put both grounds on the same plane. If this solves the problem, them look at your grounds as the issue... if it doesn't - I'd look more towards the connections, your wires, sources of interference and making sure (sheepishly) that you're connecting the right outputs to the right inputs.

Bren R.