Yea, a tube amp brings out the best in the HD650. The only problem I see with a tube amp, unless hybrid, is that they usually don't play nice with lower impedance headphones. So if you ever decided to pick up another pair of headphones, such as Denons etc. It won't sound very good through a tube amp. Now, if you are content with your HD650s, with possible sights on the HD800 or any of the Beyerdynamic series of headphons, a tube amplifier will be a great investment. For 500, I would consider: Woo Audio WA3, Bottlehead Crack+Speedball, or possibly a WA6 if you can find one for a good price used. If I were to choose between the 3, it would definitely have to be the Bottlehead Crack. An amazing amp, especially if you upgrade the tubes. Now, if the extra cost of tubes is a problem, you might want to consider a SS solution. In that case, look into the Burson HA-160, Matrix M-Stage, Yulong D100 MK II (AMP/DAC), or Schiit Asgard 2.
Now, depending on if are DIY friendly, building a Bottlehead Crack is rather inexpensive, and the kit is at a ridiculously low price for what you get. Also, if you are looking for a AIO unit, one of the best I've heard is the Yulong D100 MK II. Excellent value and punches way above its weight. It really depends on your plans for the future. If you plan on getting any low impedance headphones, don't get a tube amp. If you don't, get a tube amp.