Teansistors work very well for accurate amplification, but that's not what a guitar amp is supposed to do. It's a musical instrument, that has desirable sonic characteristics. A tube power section, when pushed into distortion, produces harmonic content that is very different from the odd-order trash you get from overdriven transistors.

Do some reading on that Ampage that pstring suggested. There are some very useful discussions of the differences.

Also, use the search feature of this site to dig up some of the older threads on this topic. I spent a lot of time discussing this stuff with other members.