For what...?
For which device; what kind of music; bluetooth or jack, what are your expectations...? Just some fundamental questions...
For n900 the stock that came with the device is quite good (for me) especially with equalizer...

Edit: I also have this and it is quite good too but has a bit of incompatibility with the jack (cannot push fully the jack or the sound gets awkward...); and I cannot hear too much difference to the stock one (I learned music at primary school).