02-07-09: CyclonicmanBingo.
My preamp takes about an hour before it sounds best too. I think the warmup period is not for the tubes, but for the other electrical components within the preamp.
I have some all-tube guitar amps, and once the tubes are up to operating temperature, they're ready. Having been a solid state hi-fi guy for decades, I was really surprised that I could immediately hear the difference when I swapped in some '60s-era NOS RCA grey glass 6V6GTs for the stock Sovteks in my all-tube (including rectifier) Top Hat combo amp. No break-in or settling in required. I think it's the SS components that take awhile to reach optimum operating temp and attendant tolerances. I keep many of my SS components on all the time for this reason.