Not a dumb question at all- in fact, if you're happy with your main speakers it only makes sense to want to incorporate them into a new HT rig. But, it generally isn't all that practical. The main problem is that the "timbre" (or basic voicing) of speakers varies a lot from brand to brand, or even from model to model. It's very unlikely that you'll be able to find a speaker that sounds similar enough to get a good voice match, and no reliable method of finding one without trying them all brute force style. You may luck out and find a serendipitous mating, but the odds are long against it.
With speakers that don't voice match well, you'll find that as sounds pan across the soundstage they'll change in tone quite noticably. A Harley might turn into a Yamaha dirt bike as it moves from the left to the center of the screen! And actor's voices will also change in sound as they move to dissimilar sounding speakers. Yech!
Also, it's my opinion that speakers have generally improved a lot over the last 20 years or so. I realize there's a lot of fans of vintage gear, and I'm not putting them down, but there's never been as much good sounding gear for as little money as there is right now. We really are in a Golden Age for HT.
I'd suggest the best thing is to start over from scratch and get 5/6 voice matched speakers and a sub. Five or six identical speakers would be ideal in theory, but in
actual practice many find that due to placement issues and a host of other concerns that specialized speakers designed for each purpose is often the best way to go. You can buy a set of towers or standmounter's for the front, a matching center channel, and a set of special surrounds and get excellent HT performance. Especially handy in many rooms is that many surrounds are made to be wall mounted. If you buy them in white, you may leap many WAF hurdles!
That's not to say you must scrap your faithful JBLs. You may want to keep them for a 2nd zone or 2nd stereo-only rig. Maybe you have a shop or office that could use some music? An entertainment room or workout area that could use a 2nd zone music system? Maybe you can give them to a relative or friend that may be building their first system? And there's always Ebay.
