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

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.

The primary reason for going with tube pre into solid state amp is that it is much easier to get a lot of power from solid state than tube circuits. While I am not a fan of solid state amps of any power rating, I am also not a fan of most high powered tube amps as well. A lot of high powered tube amps sound hard and brittle and less natural than comparably powered solid state amps.

But, when feeding low-powered tube amps, the linestage issues become more mixed. I've heard a number of tube linestages that sound good in certain setups and quite bad in others; and the differences are often unpredictable. One really must try the specific pieces together. The solid state linestages I own and have tried in several systems (Placette Active and Levinson No. 32) seem more consistent and predictable than tube linestages I have heard.

But, when you find a good tube-into-tube arrangement, there really can be magic.