Check out this review by Positive Feedback: Cyber 20
And you can also read my impressions here: My review
Both reviews used HD600. But I had a chance to try a pair of HD650/Zu hooked up to my gear. I was so in awe with the improvement over the HD600/Cardas that I purchased a pair of HD650. More details, more dynamics, better bass, and more air. I was listening to Yo-Yo Ma's "Solo", and I could hear/feel the wood texture of the cello.
I still have both pairs of headphones in my possession. I think the HD600 is probably more neutral while the HD650 is more tuned to soften up the "digital" music. I recently hooked up one of the headphone amps to my main rig and listened to my LPs through the headphones, and I prefer the HD600 over the HD650.
This amp is within your price range. And the only thing I would recommand is to spend $30 - $40 to buy a pair of GE or Sylvania 6BQ5 tubes (doesn't need to be matched) to replaced the stock Russian 6p14p tubes. The stock EI 12AU7 is ok, but of course, feel free to spend some $$$ on some NOS tubes if you can afford it.