There are way too many issues being conflated here. Tube sound=distortion is a gross over-simplification. Both tube and solid state circuits produce distortion; tube distortion tends to be even-order harmonic, which in even fairly large amounts (say, 1%) is relatively benign to the ear; solid state distortion, on the other hand, is more odd-order, which sounds amusical and unpleasant, even in very small amounts. The whole idea of a tube buffer adding warmth to a cold-sounding system is very un-high fidelity, in my opinion. Components aren't suppose to add ANYTHING to the signal, just amplify it. A perfect pre or power amp, tube or solid state, has NO sound of it's own. The better a pre or power amp is, the harder it is to tell which it is. But one thing is true: different speakers like different amplifiers. Low impedance/efficiency speakers (Maggies, for one) need an amp that delivers current. That is a solid state amp. High impedance/efficiency speakers respond to voltage, which is.....you already guessed. A tube amp. This is, of course, also a gross over-simplification. If a speaker has a wildly varying impedance (the original Quad Electrostatic, but many others), it's impedance will interact with the output impedance of every power amp it is hooked up to, and it's frequency response can be quite different with different amps. Solid state amps have much lower output impedance (i.e. high damping factor), so they react to the speaker's impedance much less than do tube amps, with their generally higher output impedance. This phenomenon is usually greatest at high frequencies, where reactive-load speakers have often have their lowest impedance (the Quad's impedance drops to around 1 ohm up there!). That is also why solid state amps are almost always better at bass reproduction than are tube amps---low output impedance/high damping factor amp driving a low-impedance woofer. John Atkinson always includes the output impedance information in his bench-test results of power amps reviewed in Stereophile.
Since pre-amps don't react to a speaker's impedance, it is a safer route to use a solid state amp and a tube pre-amp, if you think you need to have tubes in your system to get the sound you want. But some tube power amp/speaker combinations are magical together. Look for a prospective amp's output impedance, and it's prospective speaker mate's impedance, then try them together. For a pre-amp, listen to your source with and without the prospective pre's in the signal path. The one that degrades the signal least is the better pre-amp, whether tube OR solid state. We don't want to ADD warmth to our systems, we merely want it reproduced if it's in the source material. Right?
