Well, in my experience (for what it's worth, which may not be much!), there's always a certain residual background "presence" in recording. I won't say it's always a hiss, but that's the nearest I can get to explaining it. Wikipedia calls it "...ambient noise level (sometimes called background noise level, reference sound level, or room noise level) - the background sound pressure level at a given location".

So, if a recording input level is low, it may be that the difference between that recording and the accompanying ambient noise may be quite low. The effect of amplifying the recording level, say with Audacity, is to increase not only the sound but the ambient noise surrounding it. Recording at a higher level, and with the mic closer to the sound source seems to make ambient noise less of a presence. That's what my ears tell me at any rate.

I won't swear to all of this; merely to say that, when using the two Zoom H2 recorders - one of which was recording at a very low level and had to be amplified quite considerably with Audacity - the overall sound, including ambient noise, was boosted. This is how it seemed to me. I can't figure out why one of the H2 machines is losing its recording level. The settings on both machines are exactly the same - and I've checked and double-checked - but, at equal placement from the sound source, one wave is much, much lower than the other.

Regardless of all that, I'm glad I bought the H6. The ability to plug in my XLR-cabled Shure mics is certainly a bonus!