I've been using it for a couple of years until I got the Rudistor. I've had the oportunity to drive Senn 6xx, K701, DX-1000, ULE 2200.
IMO it's the paradigm of the "Flat-earth sound", which aims more to the rhythmic and PRAT aspects of the sound than to the timbric and resolutive ones. With the HD600 it manages to put the missing fun factor that these cans have when used with other amps. With the K701 it's just OK, it has power enough to drive them, but the sound is a bit weirder, as if this amp enhanced the K701's flaws.
With the Ultrasones is a very good match too, it pumps up the fun factor of the cans and makes them more enjoyable. It doesn't cover their lack of coherence nor injects any band aid to their lack of resolution, but overall they sounded better to me than when used with the X-Cans.
I didn't like the result using the DX-1000. These cans put in evidence that timbric correction isn't one of the strong points of this amp. Playing pop-rock the sound is OK, quite fun, kind of open, alive and fast, but when listening to classical or jazz, there was something weird on the tonality. Maybe it was just the amp enhancing the slightly flawed midrange on the DX-1000, I can't be sure, but the DX-1000 sound more correct to my ears on the Rudi or even on the HP output of my PC card or TV.
Overall it's a good amp, maybe overpriced for the sonic results, but considering it is also a more than decent preamp with 3 inputs, includes remote volume control, and has a very good built quality, it's a fair option.
Rgrds