I am using my Linn Majik as a preamp. I tried swapping in my Headroom Maxed Out Home to serve as a preamp (which the Headroom literature says can be used as a preamp), and the headphone amp did not come close to the Majik. This is a pretty darn serious headphone amp vs. the pre section of a one-step-above entry level integrated, so I think that's instructive, albeit it's only one data point.
I almost bought a McCormack Micro Integrated Drive (one is listed in the classifieds now), which is designed for both headphone and preamp use. I'd be very curious about that piece's preamp performance, although I wouldn't be inclined to expect too much unless the optional, heavier-duty power supply were part of the mix.