I mean hard headphones like HD800,LCD,HD6xx series or etc.Not standart headphones like Momentum.I don't know HD700 but I tried HD800 with my RWAK240 at Sennheiser store and I did not get some quality.I mean I got nothing with it.Because they are not designed for them.They are for iems.
I just don't agree with your assessment on principle alone. an expensive portable like the AK units should be able to drive *any* headphone (phoens or IEMs etc).
The plus side is that my AK 120 II works fine with my Audeze LCD2s, and they're not renowned for being easy to drive phones...I can't see the HD800s being harder to drive, but I have no experience with them. I'd also be pretty disappointed if such headphones didn't really work well.
I haven't tried recent Sennheiser phones, having only owned and used the HD540 Gold references and also the HD433 phones (and now that I think about it, PX100s)...the latter of which is dead (dead drive unit in one of the channels). I much preferred the 433s, more musical than the HD540 Gold references, which are a bit too "polite" for my tastes. The PX100s were between the 433 and 540 in sound. Many have said that the Sennheiser house sound is "polite", so they're not usually my style of phones anyway as a rule.