The Carvers are not the perfect solution but I would not hesitate to incorporate them as rear surround over the Bose any day, especially if they will no cost you anything. The Carvers go for around $150 each and will more than suffice until you decide to get matching JBL's.

I suggest a net search for timbre or timbre matching to get a better handle on why we recommend speakers from the same mfg and preferably from the same series if you were buying something new. If they are donated, then live with the compromise until you have the funds and\or get the audio bug!

Be careful, once bitten, you may find your cars spending less time on the road and more time sitting in the garage.