ahofer
I've tried a variety of preamps and the only real difference I noticed was an increased noise floor (that was with a Spectral preamp, not a tube unit, interestingly enough) and some tube bloat.
So true ahofer, that's all they can do, increase noise/distortion/colouration and give tube bloat, against going direct if it's a match, and 99% of the time it is a match.
Many tube preamps aren't a match with low input impedance amps also.
Cheers George