Slow-start is only really needed when converting an amp from tube rectification to solid state, or if the amp uses a choke-input filter. As for whether the best rectifier is tube or SS from a sonic perspective, that's a controversial subject, and the answer is further clouded by the class of amplifier. My 6V6 dual mono SE amp uses 6X5 rectifiers. The guitar amp I built last year with a pair of EL34 outputs also used tube rectification (5AR4). The 211 A2 SETs I'm currently massaging each have a hybrid supply (full wave bridge with a 5R4GYB for two of the diodes). On the other hand, I have yet to build any push-pull hi-fi amp this way. One reason for that is the fact I'm big fan of the old HK amps with doubler supplies. It is precisely the minimal amount of voltage sag that I like so much about these supplies, and that has led me to believe FW and FWB supplies should also be SS for AB1 hi-fi amplifiers. For preamps, I don't think it matters, although in the context of this thread, a tube rectifier would serve to eliminate the turn-on surge and to maintain the original operating voltages. The turn-on surge in particular is a potential problem. It means all the decoupling capacitors throughout the circuit, including those in the low level stages, must be able to survive the full B+ voltage from the power supply. The only solution, other than (usually) increasing their ratings, would be a relay that delays the application of B+ until all the tubes are ready to conduct.
Jack
