Re: Marantz PMD 661 noise floor

I'm not sure, but something does come to mind; dithering.

Dithering is the deliberate addition of a very tightly defined (and very low, basically 1 bit p-p) amount of a specific type of noise to the signal. That greatly improves quantisation errors at the expense of raising the noise floor, which has the subjective effect of reducing what you hear as noise (which sounds counter intuitive, but it's a well established technique and I've heard some tests that have been quite impressive).

I couldn't be sure, but that's my guess. TBH -79dB is pretty good and it's much more likely that if you have noise problems they'll be from background sound, mismatched equipment or electronic interference than the recording.