Output transformer designs I think should be your key consideration. You've got a broad range of impedance curves and a planar magnetic too. Prime candidates for under a grand are the Mad Ear+ and Woo audio 6. These are both great sounding output transformer coupled circuits that work well with a lot of different impedance loads.
I think you might also be able to land an earmax pro for that price too. Its an OTL design that uses feedback loop for better synergy with lower impedance loads. I have an original Earmax without the feedback loop, and run it with an upgraded 60 watt power supply. That was a HUGE step up from the crappy under-spec'd OEM wall wart. Its a great sounding circuit with all my cans... for moderate listening volumes, when I crave a lot of even order tube distortion.
I have a DV337 and its by far my best sounding circuit. It has a very layered soundstage, and spatial separation that I don't get from my Earmax, Millet hybrid, and Larocco PPA (solid state). It also adds a layer of bass boost below ~35-40 Hz, that wonderfully compensates for the natural roll off of all my cans.
I run the 337 through 20 watt speaker output transformers. (a kind of semi-mod) So the lowest impedance load seen by the 6AS7G tubes is still north of 1000 Ohms.
I tried running the earmax through through output transformers and its too cold/sterile... like a bad sounding solid state amp. So the Earmax is best left alone in that regard.
I will have some new HD650 drivers by the end of the day today to transplant into my HD600... I'll reply back with my HD650 impressions later tonight.
The K701 is a tricky headphone to amp up. You have to figure out which camp you are in. Do you like its upper midrange harmonic resonance? Or are you part of camp-B that finds it to be plastic-ey, synthetic, artificial and weird overall? Depending on the amp you chose, it will have an effect on the overall K701 sound. My worst offender is my Millet hybrid, for whatever reason it sounds really odd with the K701. Solo acoustic guitar is just "off" sounding with this pairing. The 701 I think sounds better with tube even order distortion, it tends to work well with the 701 harmonic content. The K701 amped with the 337 through output transformers sounds really really good.