Why the change of equipment?
Seems a sideways and possibly slightly backward step to me...any changes in sound quality are likely to be fairly minimal, even with revealing speakers.
Nothing super fancy but I have a pair of JBL 7's that I bought new in 1994 and just had 4 of the 8 speakers rebuilt. The mid-bass and the bass speakers surrounds had just started to wrinkle, the speaker shop told me to replace them before they start to rip. The other speaker surrounds were in great shape. They did a great job and they sound wonderful again.
Well it'll work using the Sony as a centre speaker, but it'll sound odd compared to the JBL's. In a surround setup it's desirable (or essential if you're looking for the optimum sound) for your front three speakers to have the same timbre. Anything panning across the front soundstage will sound odd otherwise as voices and effects transfer from speaker to screen to speaker. The centre speaker in a surround setup is the most important in that it does most of the heavy lifting. You'd be better off grabbing a decent centre that is as close to your JBL's as possible and hooking it up to the Denon IMO.