I'm glad you asked! The Headstage Arrow 12HE drives the HD650 extremely well.
At little contextual history - it ties in nicely. I too like portable amps. They are, simply, a more realistic option for my listening habits right now. Maybe someday (when the kids are in college?) I'll have time to sit in my arm chair with a pipe, my feet up, and watch my tubes glow. Anyway, I was looking at full-sized cans and spent half a day at 32 Ohm Audio in Portland, OR auditioning headphones. I went in with my amp, Sansa Clip+, and a ton of FLACs - everything from classical to progressive metal. I wanted to like the uber retro mahogany Grado RS-1s, or the ultra modern Beyer Dynamic T1s, but that wasn't in the cards. The HD650s sounded the best through the Arrow. Everything sounded good (except maybe the T1s ) but I kept going back to the Sennheisers, and smiling - LOTS. I could drive the crap out of them without blowing the top of my skull off.
Just today I fell in love with the pairing again. I'm normally a metal guy, but today was Mozart - all day. Most of the office was empty, so I put away my in-ear Shures, set the Arrow's gain on III (zero hiss and it drives the big 300ohm cans with room to spare) and really cranked it. It was the first time I've gotten a chance to blast classical with the Arrow+HD650 combo and it didn't disappoint. In fact, I was smiling for hours. I could hear bow taps, muffled pants and chair scuffs, but no distortion. Tons of clean power and excellent channel/balance accuracy.
I believe the Arrow's around $250 USD - a bargain in my opinion. I bought it to drive my Shure SE530s. I didn't expect it to do what it can. It's an extremely efficient and versatile amp. Check out dfkt's review (admin over at Anythingbutipod).
I just love portable amps for some reason. I've already got a Predator which I think drives the 650 pretty well. Do you guys have experience with any other portables that drive the 650 better. I'm kinda hoping to trade or sell the Pred and get something else as I don't really need the DAC functionality like I thought I did. Suggestions? Pico Slim, Protector (SE mode more than likely), SR71a, Pico are some I'm really eying.