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".

Having owned both SS and tubed amps, and SS and tubed preamps, I believe the largest change in sound comes from the amplifier. If one wants the sonic signature of tubes, i.e. extended grain-free highs, holographic image and magical midrange, I'd suggest starting with a tube amp and a SS preamp. I have never heard a SS or chip/digital amp that has the three dimensionality of a tube amp. My recommendation runs counter to commonly offered advice, but it's how I'd start over if I was building a system from scratch.

BTW, one CAN have excellent, deep, controlled bass with tube amplifiers with careful amplifier selection and proper matching with loudspeakers.