Did your car come with BOSE?
If not you are safe to use aftermarkets.
My personal favorite are the Infinity speakers, kappas if you can afford them, I've had infinitys in two of my cars and they seem to be the best bang for the buck in your pricerange without going to component speakers.
If you have BOSE?
I'm unsure, I'm not sure if the resistance on the bose speakers is 4ohm or 8oh or something screwy, I know some cars which are equipped with BOSE are PITA to mess with for multiple reasonsh bose systems, however never had a bonneville with bose (yet)
Someone who has bose and replaced the speakers can chime in. A particular member that modified his stock head unit (to work with an ipod) comes to mind but I forget if he has/had bose or even replaced the stock speakers
A bandpass box is easily going to offer you the worst sound quality just behind a regular ported box, but hey if its free what gives?
But yeah check out Infinitys for sure.