Why Tube Pre-SS amp over SS Pre- Tube amp?

The idea is SS amps are more powerful and better able to drive most speakers. So by adding tube warmth with a tubed pre you're still able to use the speaker of your choice with those SS watts. As always ymmv and all cases are not the same. Most of all use whatever floats your boat, and don't get weighed down by dogma.

Another factor in that paradigm is that tubes in a pre generally last 1000s of hours, while in many power amps, their lifetime is more limited. For that reason, cost of NOS power tubes is much higher. So total operating cost can be higher for a tube power amp than a tube pre.

For me it was experimenting w/gear back in the 80's when I came across the big SS amp/tube pre combo & it always was a good matchup. Over the yrs. I've discovered that a well implemented design, be it SS or tube, will give you the sound you desire.

I agree w/Jond "don't get weighed down by dogma" and will also add my mantra of "trust your ears".

If you want to know the technical rationale for choosing a tubed preamp and SS power amp, tubes operate at hundreds of volts and are much more efficient at voltage gain. Solid-state devices like transistors operate at lower voltages, like tens of volts, but due to their lower output impedances, are much better at driving current, which is what your loudspeakers need for bass control and high volumes. Since most voltage gain is found in preamp and power amp input and driver gain stages, tubes are a better bet for these applications, whereas for power amp output stages, SS devices are superior. This doesn't always translate into sonics that we like, note SET amps that have very high output impedances but many like the sonics of these amps. Like you noted, in most applications it is better to use a tube preamp and SS power amp, especially with loudspeakers that present difficult loads to the power amp.