There is definitely some very strong competition established in the headphone amp market. If you look at 'headamp specialists' like Woo Audio you know that the bar has been raised VERY high. (Disclaimer: I own one and love it).
There are definitely some nice boutique headphone amps out there. Woo Audio is on my short list as well.
Feature-wise, I wish for a headphone amp that has a DAC & USB input so I can listen to FLAC files while working on my computer without disturbing others. The Musical Fidelity XCan V8p has the right feature set, just not the quality of sound I desire (nor close to what I think Dan could design).
I also agree that switchable impedence matching is desirable, since I have used dynamic & electrostats, Grados, AKG's, Sennheisers all, and most pair up better in some systems than others.
I don't care whether it has tubes or not. Most of my gear does, but that would not stop me from considering a sand-based amp.
For my use, I would not prefer a headphone amp built into the pre, but if my future 36.5 preamp had one, I'd use it.
Finally, the physical form factor is not highly important to me, but I would prefer a compact unit rather than another full size box for my rack.
Hope this helps!
DF