In my experience, solid state amps always sound better after being on for a while and sound best when left on all the time. However, I obviously wouldn't leave the tube pre on all the time Is there any way around this?I've run into this as well but the solution is simple! Turn off your amp when turning on the preamp, after the preamp has warmed up then turn the amp back on. You'll find that the 'warm up' quality it has is related to component temperature and in the few seconds it will be off while the tube preamp is settling down, it won't lose that warmed-up quality.
Just for the record I should point out that with any amp/preamp combination, tube or solid state, the preamp should always be allowed to settle before turning on the amp. The biggest thumps created by a preamp that I've ever seen came from a solid state preamp- so this isn't a tubes/transistors thing, its simply a preamp thing.