What then is the rational behind tube pre + Solid power
Apart from the fact that there are many who believe that that combination can provide the best sound (and I am NOT one of them, as I indicated), and apart from the practicality/ease of use factors alluded to above, I think that perhaps the most frequently applicable rationale would be that for some speakers a solid state power amp is simply the best match.
Solid state power amps typically have lower output impedances than tube power amps, which makes them more suited to speakers that have low impedances and/or impedances which vary widely as a function of frequency.
Also, speakers with low efficiency/sensitivity may require more power (and/or current) than is practical or cost effective to obtain from a tube amp.
As I say, though, I am not in that camp. I consider amplifier-friendly impedance characteristics, and moderate-to-high efficiency, to be important criteria in speaker selection, in large part because of the resulting flexibility in choice of amplifier.
Regards,
-- Al
