OK, I'll put my unqualified neck on the chopping block...
Webnick: It's the ratios, not the absolute values involved, that matter most. Convention says that there should be a factor of at least 10X between the output impedance of the soucre component (the lower of the two) and the input impedance of the load in order to minimize LF frequency response roll-off (keep in mind that in actuallity, these impedances will vary with frequency, which is usually not specified). Again, the vast majority of preamp/amp combo's will satisfy this criteria. (The other thing that should be watched for if one is using a tube preamp having a highish output impedance, say more than a few hundred ohms, is to avoid long runs of highish-capacitance interconnect, the combination of which can cause premature HF roll-off.)
Philojet: Simplistically speaking, the differences between tubes and transistors that people talk about as far as distortion spectra go has a lot to do with the way they behave when driven into or near clipping (tubes behave more 'nicely', meaning higher overall levels of THD can be less objectionable with tubes). Below clipping, the main differences between distortion spectra have as much to with circuit design as the type of active device(s) used, although tubes will usually display higher overall distortion and noise levels. Simpler tube designs (triodes) are usually considered to display 'nicer' distortion properties than the more powerful tetrodes and pentodes, and a similar characterization is often applied to FET's as compared with bipolar transistors.
As for the distortion spectra themselves, low-order products (mostly the 2nd and 3rd harmonics) are considered less undesirable ('nicer') than higher-order harmonics, a smoothly descending harmonic series (from low-order to high-order) is considered preferable to an unevenly descending/ascending series, and even harmonics tend to be surpressed by typical push/pull amplification circuits (yielding a harmonic series that emphasizes the odd-order products, which by definition are higher-order than the even-order products, and results in an uneven roll-off of the harmonic series). This description ignores a lot of other considerations and types of distortions, including the questions of distortion levels vs. distortion qualities vs. other considerations (such as higher power capability) that may come into play as design trade-offs. (EE's forgive me...)
