hi and lo,
in engineering "unity gain" means no attenuation or amplification in a circuit block, 0dB gain so to speak. Maybe I do not grasp the tapers concept of "unity gain" because I am more of a design engineer schooled in analog and audio design than an audiophile (and also not a native english speaker). The sweet spot in digital recording as I have experienced it is to set the gain just a few dB below clipping to preserve as much resolution as possible without having to resort to too much (digital) amplification in post. Maybe in the time of 24 bit recording this is less important than back when we had portable DAT recorders with 14 bit effective resolution (yes, I am that old...).
Starting back in my analog cassette tape recording days 30some years ago, I always tried to get best S/N ratio for my recordings, since most of the time it is nature recording that changes to sound pressure levels approaching or surpassing a live rock show when the train goes by. The worst of both worlds, so to speak, without the chance to redo it (and I am not talking about biological and technical noise sources that make "concert talkers" look like a minor problem).
Now back to topic: if you get hiss in the recording when using a preamp, then there is something wrong with your gain distribution (barring mic problems or noisy phantom or plug in power). For a minimum of additional noise, the first stage has to do most of the amplification. Then the noise contribution of the following stages can be neglected. Ask any ham radio operator who does weak signal work on VHF, UHF or SHF. Its physics.
There is one other noise source that a taper friend of mine found: he forgot to turn off the plug in power of his recorder when using a mic with internal battery. The electrolytic caps in the output of the mic were reverse biased and started acting up in the form of white noise and some nasty pops. Maybe this applies here?
Hope this clarifies my last post.
Greetings,
Rainer