Mixer as amp for Sennheiser HD650's?

I consider anything with a headphone jack and an output impedance of at least 150 ohms to be "an amp with power" necessary for the higher impedance cans.  As far as they sound, IMHO, you get what you pay for.  If you're going to buck up on some nice cans, don't starve them with cheap alternatives for power.  I think a moderately priced set of cans driven my a moderately priced amp will sound better than expensive cans powered by a cheap amp.  When I put my my DT880s into my Sony Stereo headphone out, they sound dull, flat, tinny, and lifeless.  Coming from my Woo amp, they are alive and wonderful.  JMHO, YMMV.

Personally, if I had the 650s, I wouldn't consider spending less than what the Asgard costs.  There are lots of posts here that say the Asgard does very well with the Senn 600 and 650.  A new batch of Asgards is coming out at the end of the month, and with Schiit's great return policy, you can probably buy a B-stock Asgard for $200 at the end of Sept.

Didn't have good experience with Behringer gear headphone outputs. Their line-level signal path is usually decent, but they tend to skimp on power supplies and headphone amp stages.

I use Mackie Onyx 820i Firewire interface/mixer with HD650 and I'm pretty happy with it. IMHO Onyx beats Benchmark DAC1 on sound quality, while being less than 1/2 the price, but this is my personal opinion of course.

I also use it as pre-amp and ADC for my Shure SM7B microphone (very accurate and well shielded full size dynamic mic, the model that Michael Jackson used to record vocals for the Thriller, its naturalness startles people I talk to on Skype:-).

I like the neutrality and flexibility of a professional mixer. Basically, it is a complete and highly tunable sound input/output center in one rugged well shielded package, with massive and very clean power supply. 

But then again, I use studio quality 32/96 digital equalization running on computer prior to outputting the digital 24/96 stream to Onyx, so I can simulate any frequency response curve or add tube-like distortions when I like, with inaudible (to me) levels of unwanted distortions. This is an integral part of the package I'm currently satisfied with.

A high-end professional mixer with equalization knobs set to zero is close to a "wire with gain", and some people don't like it, preferring amplifiers with "character" (tubes, Rudistor etc.). Still, nobody would preclude you from turning those knobs :-) In my experience, HD650 is in need of a moderate bass boost and slight treble boost if your comfortable listening level is low, as mine is.