I've heard enough tube and ss amps to realize that's it's virtually impossible to make generalizations like the ones listed by Freddy (no offense to Freddy). Each amp is different, has a different design, interacts with different phones differently. For example, to say all tube amps have inadequate bass is to ignore the Ear Max Pro, a very simple all tube design that is widely known for its incredible bass response when used with the HD600.
There's just no short-cut to doing the homework on all the amps in your price range. It's way too simplistic to rule out *all* ss or *all* tube amps based on some stereotypical differences. Also, diffrences between tube and ss amps are not as pronounced in a headphone environment than they are with speakers.
Finally, tubes wear out and will need to be replaced. Also, be aware that to get the most out of a tube amp, you will ultimately want to venture into the shady and expensive world of NOS tubes. On the plus side, tube-rolling allows you to somewhat "season" your amp to taste, whereas with a ss amp, if you aren't in love with its sound, there's not much you can do about it.
markl