IME HD600 pairs well with either (SS or tube). The AKG's really like more current than voltage - so either a hybrid or SS.
Worth looking at (IMO) is the Audio-gd NFB-15.32 (http://www.audio-gd.com/Pro/Headphoneamp/NFB1532/NFB15.32EN.htm)
For the price - considering the very powerful amp, and very well implemented DAC section - it's a good value proposition. I love it with my HD600 and actually preferred it (over my LD MKIV) for the AKG's (K701, K702 & Q701) when i owned them.
Which one better do you think? Audio GD NFB 15.32 or Bottlehead Crack? I don't know if this is a fair comparison for a DAC cum amp compared with amp.