Question: Why do people use...

Tubed preamps and solid state amps as opposed to SS preamps and tubed amps? Is there more going on sonically in the pre than the amp?
I haven't used this setup (yet), but the idea is to combine the power and low frequency abilities of a ss amp with all of the attributes tubes are known for. Seems like a great idea to me and I plan to try it at some point. To reverse the set up and have a ss pre and a tube amp would miss the point.
High gain amplification of low voltage signals is something that tubes are good at. High current low gain amplification is something that transistors are good at.

Can it be done the other way? Sure, but it isn't easy and will cost you.
I've auditioned all setups and still found Tube preamp to tube amp most appealing sound by far. Tube preamp with SS amp is the runner up.

SONY ES9000 SS preamp with Bryston 9BST SS amp
Audio Research SP-11 tube preamp with Bryston 4BST SS amp
ARC sp-11 tube preamp with McIntosh MC-240 tube amp
Mark Levinson 23.5 SS preamp with Sonic Frontier tube amp
Mark Levinson 23.5 with Nelson PASS X-150 SS amp
Marantz 7c tube preamp with Sonic Frontier tube amp
Marantz 7c tube preamp with Classe DR-3 HVC SS amp
Counterpoint SA-5000 with Lafayette KT-550 tube amp
Counterpoint SA-5000 with SET EL84 tube amp

I agree that most of SS amp has better low frequency reproduction but there are many tube amp can also produce good low frequency reproduction. I don't think BASS would an issue in tube gear as long as your amp is within spec.
However, tube amp are not recommended for Subwoofers because of the constant high current demand.
As the user of a solid-state preamp and vacuum tube amps, I have a different take on this. Since this is the reference system for my audio company, Virtual Image, I chose this particular solid-state preamp because, other than a slight leanness, it is extremely neutral, as a reference should be (the fact that I was able to eliminate that leanness later is icing on the cake). Solid-state amps, with their high damping factors, tend to offer overdamped bass--dry and unnatural-sounding, IMO. An extreme example of this was a friend who had Klipsch Cornwalls and drove them with a Crown DC-300A ss amp (known in the 70s for its high damping factor). We thought the speakers had no bass, but the problem was that the Crown amp was telling the 15" woofer to stop before it could overcome inertia and get all that mass to move! He bought a used tube amp, and, voila! Low frequencies!

It's an unfortunate dilemma of life and physics that a low output impedance is necessary to eliminate frequency response variations, but that it strangles the bass via overdamping. You have to accept the compromise that works best for you in this regard. As for me, I'll take slightly underdamped over slightly overdamped anytime (until perfect bass becomes an option). Obviously, some tube amps are waaayy underdamped, and I find this as unacceptable as solid-state bass. As a learning example all you need to do is go to a live concert, pay careful attention to the bass, go home, listen to a live LP or CD of similar music on your stereo, and cry. BTW, once you become aware of the overdamped-bass problem, you realize it's not just a problem in the bass, but in all frequency ranges.

I have a Conrad Johnson PV-7 preamp that I want to couple with SS as well as tube amps. I want to put a Bryston SP1.7 in between the CJ and the amps. The CJ is desirable to me for the phono stage and also to impart a "tubey" sound before it gets to the Bryston's neutral analog passthrough. The SS amps will be driving the home theater arrangement and, depending on my mood and the music to be played, I want to switch to tubes for the fronts. Is this at all practical? Aside from switching out some IC's and speaker cable?

My speakers are Thiel CS2.4's. I am looking to purchase the Musical Fidelity Tri-Vista 21 DAC when it comes out in a few weeks. I haven't decided on a TT or digital front end yet. Amps will probably be Bryston or Theta for SS and I have a Dynaco Stereo 70 that I am going to mod for the Thiels (for the time being until I can afford more power).

Thanks for any more input :)
Not practical becuase it will not sound the same. Better off switching out the ICs to make it sound right. A friend of my did similar thing and didn't realize why it sound bad until he remove the extra link.