I've always been told to start with a strong signal and cut progressively through the signal chain so as to avoid adding noise at each stage. So my mixer usually has a pretty high trim setting and I shut off anything not absolutely necessary to run raw signal into the computer. I'm dealing with borderline equipment, so it's a challenge and sometimes the track is noisy as the dickens.
But if the preamps are kicking in with greater noise after a certain attenuation level, maybe there's a way around it. I've found the virtual preamps in the n-track effects units are pretty good. So one experiment might be to run signal into a mixer with with the channel trims down a bit ( like a third or so ) and see whether boosting signal with a reverb or short echo virtual preamp later in the chain improves the sound floor or makes it worse.
I've also been experimenting with volume envelopes in n-track to cut any signal not necessary for a mix. It's a pain but it helps. I haven't learned much about compression yet (other than reading everything here I can find!!) and that'll be the next thing to tackle.
But I guess there's just no substitute for a good mic and a good preamp, any way you look at it...
