My experience with the k701/Headfive combo was satisfactory, as far as driving them went. The soundstage was large, the highs sparkly and detailed, and the mids truly something to hear. IIRC, the volume pot went around 9 o'clock, with my x-fi's volume at 100%(as recommended by Dr. Meier).
I sold the k701 as I preferred the dt880's more natural soundstage and heftier bass. I sold the Headfive to fund a video card purchase
I'm now using a Pioneer VSX-305 stereo receiver, which imo is an upgrade as far as functions go(surround sound modes, integrated bass and treble controls, etc). It also has a LOT more juice(current, voltage?) to drive any headphone I've used so far, as the volume pot is a LOT bigger, and the sound gets proportionally louder as I turn the knob.
I once tried the H5 of one of my german buddies with my MS2, the DT880 and with his K701. I liked the sheer tone, but the soundscape abilities were really poor. Guess that is because Jan chose a cosy groovelizer Opamp instead of some high-res soundscape monster such as OP627 or LM4562.