Well your rears are completely different from the rest of your setup so they aren't timbre matched to begin with. Just because they are towers and your fronts are towers doesn't mean they sound even remotely the same, which I'll bet the house they don't given that the design goals of CV are completely different from the design goals of the Onix speakers.
Since you're crossing the rear surrounds over to a sub, towers aren't necessary, only a good pair of bookshelves. The rear speakers can work pretty hard during the right movies, but a great set of bookshelves would be able to handle that easily, especially if they're crossed over to subwoofers. Plus you'll get a better return for your money with bookshelves, unless you're budget is above $1000.
Also, what is your budget?