Is upgrading pre worth it?

You're not hearing what your amp is capable of. I agree with others that the preamp has more to do with the sound of a system over an amp; they're both important. I've heard ok amps sound very good with a great preamp, but not a great amp sound great with an ok preamp. Try the Rogue pre... can't go wrong with that brand.

Most definitely yes.

 The pre-amp is the heart of the system..

Wow.. ok learning something here. NOW.. rogue has an the pre99 super magnum. that would be the equel to my st100. what would be a good soild state that might be better. IS it ok to mix solid state pre and tube amp of different brands?

kingdombuildingcon it's no problem mixing ss and tubes assuming there are no impedance mismatches or anything but I think, and others here too, that a tubed preamp is the way to go. In fact many do the reverse of what you propose and pair a tubed preamp with a ss amp. I would say unless you have some compelling reason to get a ss preamp stick with tubes.