Pass Labs X2.5. You'd have to find one used. The newer Pass preamps do not have a balanced loop out.
http://www.passlabs.com/pdf/old%20product%20manuals/x2.5_om.pdf
EDIT: Looks like the Aleph-P would work too. Those come up on A'gon quite often.
http://www.hififever.com/forum/files/apman_134.pdf
Hired a bodyguard for the wife. Kinda weird. Former South African special forces blah blah blah. If it's going to turn out like The Bodyguard, I hope he's been lifting weights. And I simultaneously deny that I ever said that.
Here's a dumb question, but where is the flat bit of the otherwise circular O2 pad actually supposed to be oriented relative to the outside of one's ear?