The last couple of weeks I've been listening to my HD650 on three different setups:
1.The Burson Audio Conductor V2+. Music from MediaMonkey on my laptop.
2. Garage 1217 Project Ember fed by the DAC direct output on the Burson. Music from MediaMonkey on my laptop.
3. A pair of Nuforce HA-200 set up as balanced monoblocks and fed by a Advance Acoustic mda-503 DAC. Music from Shinrico digital media player.
To start with all these setups are powerful enough to drive most headphones and all of them have more than enough power for the HD650.
Furthermore they're all very good sounding with the HD650 and I really like all three setups a lot and they all make the best pairing with one or more of my headphones.
The Burson setup is just a pure bliss, excellent clarity, beste soundstage out of all three amplifiers and not the least the best bass. The Burson does actually manage to make the bass on the HD650 sound fairly tight, despite its slightly wooly character. The Burson does have a quite unique combination of dynamics and air in its presentation that fit the HD650 very well. This is also my favorite amplifier for the Fidelio X2, Meze 99 Classics and Q701's.
The Project Ember (with a 1950's RCA 6SN7 tube) does also perform very well with the HD650 and is not far behind the Burson I it's overall performance. I prefer this combo with the Ember in its 0.1 Ohm output setting but even then is the bass looser than on the Burson and it also lack some air in comparison. This is my favorite amplifier with the HD700.
The AA/HA-200 combo does also sound very nice with the HD650 but despite this is my least favorite combo out of the three tested here. The HA-200 is very smooth and laid back while still remaining highly detailed but in total this pairing lack some energy and drive for my preference. The AA/HA-200 combo is my favorite pairing with the HE400i and SR325is.
After having owned the HD650 for a couple of months now I'm still a bit torn about them. The wooly bass and rolled off treble does not suit my preference well and it does actually make them a bit fatiguing (or dare I say boring) for me with some music. On the other hand male vocals and string instruments are just absolutely sublime and the timbre is so incredibly wonderful. Music like Eric Clapton, Mark Knopfler but also Tears for Fears have never sounded better to me. While I've got several other headphones that are better all-round performers for my preference none of them can beat the midrange of the HD650 and I'm a midrange lover so this means that I value them a lot.
Just my 2c on these classic headphones