If I'm looking for "Tube Sound", why would I put a Tube Pre in front of a SS amp?

Another rationale for using a tubed preamp is that tubes work better than solid-state in amplifying voltages since tubes work at hundreds of volts while SS works at tens of volts.  Solid-state work better at amplifying current since they have much lower output impedances, so there is a good reason to use SS power amps.  Not sure which will have the greatest "tube" sound since that is more subjective and affected by so many factors.

Having a fantastic tubed front end is the way to go if you like to spin vinyl and your speakers are a bit on the not sensitive side or not tube friendly. This is the way you inject that awesome tube sound without using a tube amplifier :-)

Matt M

In my experience, all things being equal, a tube amp will impart more of a tube sound to the presentation than a tube preamp, but some speakers need ss power to drive them and in that case there is only one way to go.

There are also hybrid power amps, Lamm and Ypsilon monoblocks come to mind. Lamm uses one tube. Of course, he also makes SETs and his preamp and phono stages are all tube. 
Perhaps one day they will invent something that will sound better than both in every way, perhaps not.

Agree with Eric's explanation, electrical compatibility between the amplifier and speaker IS most important as is the output impedance of the preamp being ideally suited to the amplifier. Beyond those two very important parameters much of the rest regarding some of the other comments is subjective. I would further comment that generally tubes are more ideal for use with a speaker having a higher and more stable impedance however there are tube amplifiers with quite low output impedances that can work with speakers with more difficult loads.

Tube sound these days varies quite a bit but in my experience I generally hear the effects of tubes to a greater extent when using a tube amplifier. Using a tube preamplifier with a quality SS amplifier, well the character of the SS electronics always reveals, to me at least, that I am listening to SS. Nothing wrong with that as each have their particular virtues and that is why no one can specifically answer your question but you when listening to a specific setup.