Well, I didn't consider the Q500 bright until I plugged in the LS50s. The Q500 have a "bigger" sound, it fills the room more so - as a floorstander this is what you would expect. The LS50 produces a more natural, realistic sound. You hear textures in the music that you don't notice with the Q500; for example, if someone is singing with a rough/coarse voice you can hear the "graininess". I think the LS50 does favour more vocal or acoustic oriented music, whereas the Q500 likes to party.
In terms of bass, it is louder with the Q500 but less well defined, perhaps a little flabby. The LS50 bass is more taught and well defined. They do still produce a really good sub bass (despite what other forum posters reckon). I was listening to some Adam F and LTJ Bukem (drum 'n' bass - if you don't know who they are!) and there was a really good sub bass sound there.