Hi rossage,
.... hmmm, this is interesting. You can't be certain what is going to be driving your headphone amp, can you ?
It might be a single-ended source or a balanced one. You might need both in that case.
Not an easy thing to find on a ready-made headphone amp. I suggest you take a look at a small mixer for this.
You'd then have balanced inputs and also single-ended inputs, and you'd also get a headphone monitor output.
You don't really need hi-fi for this, do you ? So a mixer would probably be O.K.
How about something like a Soundcraft Compact 4?
www.soundcraft.com
edit: You might also look at
www.behringer.com.
There might be something in the 'mini' range that would work for you