The size of the driver has no impact (ok, not NO, but very little) on the timbre and sound of the speaker. With such old speakers it's probably going to be impossible to get a matched center (they probably didn't have centers back then). Your best bet is hunting for a speaker from CV or even from another brand that closely matches the sound of your mains.
It's great that you love em, so don't let me(or anyone else) tell you otherwise, because it's always YOUR ears that matter.
As for why they aren't too "high-end", they are mostly cheap, decent enough, and can sound fine for rock and the like, but IMO(big emphasis on the my OPINION part
) they really don't stand up to the detail and quality of sound that some other brands dish out. They are good at being loud and still sounding good, they aren't ripoffs like bose, but they aren't "audiophile" speakers either, so they don't get reviewed in the big mags(they also aren't pushing extremely expensive wares either, which the magazines tend to like, BIG money products...)
So if you've done a lot of listening to the many highly-regarded brands out there and still love the CVs, there's no reason to get rid of them. A speaker that sounds great, always sound great. If, however, you've not set foot in a high-end store, or listened to a good 2-channel setup since you've bought these 12 years ago, you might wander in, what was the VERY best many years ago can be had for very reasonable prices nowadays.
And lastly, this is all based on my impression of CURRENT CV speakers, I am totally unfamiliar with old models, so maybe i'm just clueless and these are and have been fine speakers all along.
So anyway, just remember that the size of the driver really has nothing to do with the sound of the speaker(apart from the gross differences between, say a tweeter and a large woofer). Speakers made with IDENTICAL drivers can sound totally different based on cabinet and crossover. And obviously, all 6.5in drivers, or 12in drivers, or whatever, do not sound the same, and DEFINITELY won't sound the same in different speakers. You really want to hunt by ear, disregarding the type/size of drivers, for something that matches your mains. Of course, if you decide to replace your CVs, then it's easier to buy a matched set across the front, but would cost more $$, and you'd be parting with speakers that you seem to love.
Good luck!
