^^^ I agree that most (decent) preamps will sound very similar.
What I have noticed, however, is much of the lower-end gear can have differences, mainly in clarity/level of detail. A good example is the regular Edirol UA5 vs. a modded one. I attribute these differences to trying to run standard opamps (eg., NJM, NE5532, or similar) at lower voltages, like +/-5V, or even just single sided +5V supplies. Many chips are not even spec'd down to that level! Compare that to mixers. Even the lowly Behringer run their chips at +-15V supplies. I believe it takes significant attention to detail to get great sound from "USB level" systems, either stepping up the voltage inside, or by choosing opamps that work well at lower voltage levels. The most challenging is running at 3 or even 2.5V, like flash recorders use. Compare the self noise on a well-made circuit, like Sony PCM series to Zoom, etc.
Richard