The only real single advantage of solid state over tubes is portability. You can make a nice portable with solid state devices. But not really from tubes.
Otherwise, you have to consider the headphones you will be using, and what type of sound you like. Personal preference.
If I was going to stick to my Sony MDR7506's or Sennheiser HD570's my old Creek OBH11 would be tolerable, the Headroom Supreme nice. But with the HD600's, neither of these two worked for me, for what I wanted.
So I got the MG Head OTL. Much happier with this. In this case, the high impedence output of the MG Head made the HD600's tolerable. Then there is the "Tube sound", which is ok. If anyone made a solid state amp with high impedence drive at a reasonable price I would have preferred that.
All a matter of what you want, and what you can afford.