Em, there's no good reason not to try out those other speakers as back surrounds. A rated impedance of 6 ohms(with much of the frequency range likely well above that)causes no problem with any modern HT receiver.
The surround modes I suggest are DPLII(which I always use)and Neo:6.
thanks for the info JohnK. I will go ahead and try them then. It's a smallish early 90's pair of JBLs. I like them enough to not get rid of them.
Also, so far I've used Neo:6 pretty much. Some sounds better in the Neo:6 Music and some in the Neo:6 Cinema.