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

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.

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.

I agree with Tvad. Much depends on the specific character of the components you're considering. Of all the combos I've tried, I'm happiest with my SS preamp driving a PX-25 tube amp. While the tube/SS pairing is touted as being more likely to be successful, nothing beats figuring out what your sonic preferences are and evaluating specific combinations from that point of view. Following a "generally applicable" approach isn't helpful if the particular combination you're evaluating doesn't float your boat.

Rlawry

Not an engineer so bear with me. But is that why the MOSEFT (sp?) design which amplifies voltage are supposed to sound similar to tubes.

I think plinius and belles use this circuit.

Tz&- Mosfet (sp correct) is a type of power transistor, not a circuit type.