okay, i had to give it a shot...
a. "just clean preamp." In the best of all possible worlds, an ultra-clean preamp would basically be an extremely pure wire that added gain. So your microphone would sound exactly like your microphone, except louder. This is important because microphones are extremely "quiet" in the relative scheme of things. In my experience, I have never heard a "just clean" preamp. Every preamp has a "sound," unless it's specifically designed to be pure, for example, for audio testing purposes. In that case, usually 99.99% of us cannot afford to buy it.
b. "preamp with compression." I would add to this, preamp and compression with makeup gain, because if you just add compression, things get a lot quieter. Sounds like the clean preamp, but louder, and maybe a little bizzare depending on the source you put into it. With vocals, compression can actually be a little hard to hear, except sibilance and breath sounds start to be a lot more noticeable, along with everything getting a little more even. If you've ever heard Tori Amos, and the way her breathing and whispers becomes an integral part of a song, well, that's pretty heavy compression.
c. "preamp with tube color added." This varies. Can sound really nice and smooth, can sound exactly like the clean preamp, can sound thick and dark, can sound extremely distorted. pretty much runs the gamut, depending on the tube, where it is in the chain (mic, preamp, compressor, other processor). Some tubes are extremely transparent, some are crunchy. Mic tubes:
the Neumann U47 sounds beautiful, very airy and warm, but also a little dense.
The Rode NTK (a lot cheaper) sound thick and a little dark, can sound almost distorted. Generally it's a heavier sound, but not always. Preamp tubes: an ART tube MP preamp, when cranked, sounds distorted. Other tubes I am not so familiar with. In general, most designs are engineered to add even-order harmonics to a sound, which is a fancy engineering way to say, distortion that human ears like.
d. "both b&c." More of what c sounds like. so louder, less spiky, more balanced, and probalby a little more distorted, again depending on the design of the system.
