Most solid state amps today, even the el-cheapo ones, are pretty much dead flat freq curves, non audible distortion (below 1% usually) and have very low output impedances, along with very high input impedances.

Tube amps: ahhh...they are all different. For example, I did a review of the Stoetkit Jr with the Zen se84c, and the zen se84c seemed a lot clearer, probably becuase it emphasized the midrange more (in other words, not a flat freq curve). Usually, SETs have higher (much higher) distortion than even push pull tube amps, and as they clip this sound can be liekd by some people (esp. guitar players!). Finally, having a low damping factor can artificially increase the bass in single driver speakers, esp. with heavier cones. That is why these speakers sound better with SETs than SS amps.

Which offers greater "fidelity"? I think SS amps for sure. Which is more fun to play with, as long as you don;t care about fidelity, but just want to have fun and listen to music? Tube amps, esp. SETs!

My 2 cents.FYI, I own 3 SETs: one zen se84c, one asusa (el 84 with 12ax7 drivers) and one classic SET 45 tubed (the one I listen to most, and the one that probably measures the worst!)
I also have a solid state Yamaha amp I just bought from the late 1970's: sounds great but no character: kinda like buying a HOnda versus an old MG roadster, if you know what I mean!
-akhilesh