The type and number of tubes in a particular unit is more of a design decision than anything else. My Audio Research power amp has a virtual forest of tubes, yet it could easily make do with less if they had wanted. Some people like to "tube roll", trying different (compatible) tube types to achieve the sound they like. You don't have to do this however to get good sound.
Using a tube preamp with the 950 (or any other such device) is pretty easy. I take the left and right line outputs of my 950 and route them into an "aux" input of my tube preamp. This tube preamp also has inputs for my turntable, reel-to-reel tape recorders, CD player etc. In home theater mode, I switch to that "aux" input for the 950. The one thing to keep in mind with this setup however is that you need to mark off a "calibrated" setting on the tube preamp's volume control and return to it when in HT mode, otherwise the speaker level calibrations will be off. The placement of this mark is not important, as long as you return to it when playing through the HT preamp. When the tube preamp is not switched to the HT system's "aux" input, it operates like an ordinary stereo preamp (it's always connected to the main front left and right power amps).