I have the Grace 901 and have been very happy with it. It's compact, looks nice and sounds great. Combined with a zero sampling tubed DAC it's heaven. The Sugden is also supposed to be good, but I've not heard it. Blue Circle will have one this summer. You can also check out Positive Feedback Online for reviews of an Opera headphone amp and some Ray Samuels amps. I've tried the XP-7 and it's good too, but I prefer the Grace 901. A 901 just sold here for around $900 used, but it's hard to find them used.
Before I spend 700-1500 on an HR Max (I just sold one) I would try the Berning microzotl, which can also act as a one source integrated and can easily drive 90 dB speakers in a small room. New its about $800 (hard to find used, but should set you back $500-600). Highly recommended with the 650s and some Teles and some good NOS 6SN7s (I'm using Sylvania chrome domes).
Thanks for the responses. I've been sucked into a vortex of insanity over at head-fi. I'm now learning about all these custom-made esoteric little gems like the SinglePower, the Larocca Audio PPA or something, the Gilmore V2, the Emmaline something or other, and the German RKV that looks awesome, but one member posted pictures of his after it burnt up somehow. I wonder if that's a common complaint with the RKV? Swampwalker, I know you're a regular over at head-fi, I saw your name on a lot of threads, what do you know about these other, custom amps? The MicroZotl is another one, and I've seen it come up for sale here from time to time. I'll probably give one of these lesser-known, tweaked jobs a try before investing in a new Headroom, given all I've read so far. Too bad I only have two ears, and one head.
The best? If you like tubes, then the best is a Cary 300sei (a lot of bucks!). Now, this is true for me; to my ears. Next best? Earmax Pro.
The Grace sounds fine but it is definitely solid state. The internal dac is not a very good one (sound-wise). The very high end HR Max stuff does sound very good (better than the grace) but also sounds solid state and costs way too much money. Just my two cents.
The Cary CAD300SEI is a killer headphone amp... definitely overkill on the high end of the scale $-wise, but excellent sounding. it and Senn 600s Kicked the butt of a Stax set up I had on audition for a while. Doubles as a great little integrated for small to medium room systems with speakers of 87db/1wm sensitivity or better. Add a CD player and it's a two-box system.
If you like tubes, try the Earmax Pro. I've been very satisfied with my Pro/Senn 600s. The tube combination I've found to my liking is Telefunken 12AT7 and Bugle Boy 6DJ8's. I could listen for hours (and do)!
Try the Musical Fidelity V3 . It is amazing! and under $ 400.00
Just my $.02 here, but since you didn't limit the price, I agree with Ed and Jim that the Cary SEI is nice. I disagree that the Earmax is next best though. I had the Earmax and it was (in my tastes and in my system) not any better that the headphone jack on my Pioner Elite CD recorder. I also have a Headroom Cosmic which easily bested the Earmax. My main headphone rig, like the Cary SEI, doubles as a very nice SET integrated - the Moth Audio SI-2A3. If you like a typical warmer but clean and detailed SET tube sound, the Moth is fabulous. If you like a little more slam then the Cosmic has more of a solid state sound and is tighter in the bass with nice dynamics.
I also had the first version of the ASL MG-Head amp and, although I liked the sound, it cannot drive the Senns to any decent volume and the Moth is much more dynamic, with better detail and focus.
If you like the tube sound, I recommend checking out the ASL OTL32 headamp/preamp or the ASL Twinhead headamp/pre or the RKV. If you want solid state, I think you'd be hard pressed to find anything better than a PPA amp, regardless of price. Several head-fi memebers said they heard a PPA amp with the HD650 at a meet and it blew away the Headroom Blockhead. So you could go with a PPA and save quite a bit of money. There are tons of amps out there, so be careful and don't lose your head over this. BTW I have the HD580 with MG Head OTL and I think it is a wonderful combo.
Sarah- the folks over at head-fi are often younger and/or really into DIY and specs. There is a lot of discussion about rolling op-amps, and how something must sound better 'cause it it has bigger tubes and heat sinks and all. Some of the serious folks there have very good ears, but there is a lot of chaff to separate out. I have not heard Singlepower, or PPA. Gilmore amps are designed by Kevin Gilmore, who is an academic type who has released several headphone amps designs into the public domain, and some of them are being "manufactured" by folks you can find on Head-fi. Each one is typically different, tweaked with diff caps, or op-amps or whatever. I agree that the RKV looks awesome, but I'venever been able to hear one. One member owns a Moth SI-2A2 and says Moth will customize those for your particular cans. He has also heard a prototype of a new amp the owner of Moth will be selling under another company name later this month for about $400, which he says is quite remarkable. I have heard one of the Emmeline amps and it was quite good but not remarkable. Email me and I'll give you the name of the guy to get more info on the new moth amp, which he auditioned with 650s, I belive.
I bought a pair of HD 600's with my Benchmark DAC-1 and am very happy with it. The benefits obviously being that for the 850.00 you get a very high quality DAC.
I am forced to chime in with my opinion on the Earmax Pro. I had a Headroom Max (SS amp) and the Earmax Pro is simply superior with HD650s--more liquid sound, less fatigue (crossfeed notwithstanding), amazing bass.
Be aware that the Earmax PRO is NOT the same as the Earmax--the Pro version can drive some of the more difficult headphones that the standard Earmax cannot drive effectively.
The big picture in the centre of this site:
shows a Larocco amplifier which was designed to use with HD 650s. It's off PPA design.
Click on the picture for more pics and details.
Ive used my rkv with my hd600s for four years now, sounds fantastic but I cant compare to anything else as I havent heard any other amps. Ive never felt on the other hand that anything was wrong so never thought about upgrading. Its worked wihout fault for four years but I think I should look for some new tubes for a possible upgrade.
I bought an RKV secondhand a few months ago, no problems to date. Remarkably immune to tuberolling. Seems like a sturdy piece of equipment to me.