Hi adysuzixxx,
The only thing that I would say is that 6x9 speakers tend to work better if you are using an additional external amplifier to run them. As these speakers are physically bigger than most, if you are only using the amplifier inside the BT2900 then to be honest this would be a bit underpowered and you wouldn't really be getting the best out of them. These type of speakers handle a bit more bass, but without sufficient power to run them properly you would almost be better choosing smaller speakers from a sound quality point of view if you weren't going to have an extra amplifier.
Personally if I wanted to use 6x9 speakers I would choose a suitable amplifier and then match speakers to it in terms of their power handling capability. Which ones are best really boils down to those that give you the style of sound you want. I think I'd find a good local car radio specialist, as a lot will have demonstration facilities for you to hear a selection of different speakers (as well as good advice about what might work best in your particular car).
Hope this helps.
