I wouldn't suggest you go out and buy $80 worth of music to audition headphones. I'd suggest that you seek out 5 of your favourite CDs in your collection to take with you. It might be interesting to see what Choral Pipe Organs sound like on headphones, but will the headphones be broken in or just plain broken when you listen to them? So just take a wide range of music with you: whatever you like the most and which has a wide range of instruments with the ocassional solo piece.
I prefer to listen to female vocals, so I always take Leann Rymes' "Blue". For male vocals I take AHA's "Living a Boy's Adventure Tale", although I will also take some Garth Brooks. I also like Pink Floyd, and some bluegrass, disco, rock guitar, Tibetan Monks, trance, rap, eurodance (Shiny Disco Ball), chamber, symphony, swing, big band, Dean Martin, Frank Sinatra, Jo Stafford, ...
see. just take your favourites. if you're into Jazz, take some Jazz. I prefer Jazz Piano, myself. If you love the Beatles, take some of their most demanding works with you. Take something that has a lot of bass, a lot of cymbals, pianos, clarinets, bass guitar, electric guitar, kettle drums, violins, oboes, harpisord, pipe organs, horns, trumpets, ...
and since she's going with you, see what she has in her collection.
make sure that you try to take one 'live' album with you. and if you happen to both share a favourite song... just don't be surprised if you end up buying two sets of headphones
and if you're really serious in spending money - head over to the head-fi music section, pronto. It's where I learned of Joshua Bell: Sibelius Violin Concertos and Diana Krall.