Is upgrading pre worth it?

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.

The preamp is the heart of a system and absolutely required for an excellent system.