Agree with Bill.
I think you're making WAY too much of the potential differences between VP180v3 and VP180v4 relative to integration with your existing M80's. The frequency response differences inherent in a horizontal v. vertical speaker are going to be more meaningful than the version change. Until you have three identical speakers across the front, you're always going to have some response differences. And - even then - your mains will have different boundary affects than the center.
So just get the subs and the VP180. If you really thought that your current subs were "sufficient", you wouldn't be talking about buying new ones every week.
