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

I'm purposefully not specifying speakers as I would like to know in general terms the arguments for and against. So, to the degree that speaker characteristics could also influence the choice, please  explain.  FWIW, I do not know a phase angle from an impedance curve so I will try and educate myself as we go along.  Hopefully, others may benefit too.
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.
Want great tube sound? You'll need good speakers and a good tube  amp and tube preamp. When matched right, the AMP will make the biggest difference in sound. If you like tube sound, you may not go back to SS.
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I have been a Spectral guy for 20yrs but am now parting ways. Spent time with new Spectral 30SV. Was "magical" for (1) week and then nothing but problems. I have owned a Manley Steelhead since '07 and it sounded good but used my Pass Xono more. Spectral gear is now for sale; so I pulled out a pair of Forte' 75w CL A monos and drove them directly with the Steelhead. Finally realize that the Steelhead IS a Great tubed phonostage! The Fortes' just don't have the power to drive my Avalons. Tommorow I will receive a pair of Pass 160.8s and drive them with the Steelhead directly. Also intend to "roll" my first tubes in the Steelhead. Will start to be able to weigh in on this discussion . Also have an audition of all CAT system in near future. I am rather new to "Forums" but Agon has been interesting, informative ,thought provoking and entertaining ! Keep it Up; guys and gals Thanks