The HF5 straight out of my Squeezebox Touch which, by the way, has a very competent DAC and amp built in. The amp wont drive the Sennheisers or planars obviously, but its more than enough for sensitive IEMs and portable headphones.
My HD800's are driven by my Pioneer Elite SC-25 AVRs headphone jack. Another surprising gem with plenty of power and rare (for the segment) low output impedance. I havent heard another solid state amp that can best it.
Ever try driving them out of the speaker outs? The HF5's should love it. (just make sure the volume is all the way down when you plug in)