I've got a pair of HBS-250 a2dp headphones by LG. I got them for $20 during a woot.com wootoff and they work pretty good. They have a built in equalizer with 5 presets. I personally like the superbass function on the headphones as it can really pump some for their size. While music sounds great, My only gripe is that you can hear a very small hiss (for example, when listening to classical music, etc.) but it soon becomes forgettable. The headset has its own volume controls (they do not control the N900's volume) so I suppose that's welcomed. The headphones have built in controls so you can pause, forward, reverse, etc.

Also handy feature is that you can switch between headset and headphones seamlessly so when you get a call it switches to headset (still stereo) then switches back when you're done. I suppose this is true for all a2dp headsets though.