What about prime towers with ultra center and ultra surrounds?
Since your asking I'll throw it out there....
Ultras are for someone who doesn't have a budget and wants the SVS top of the line. I fell onto mine by chance. When I did they Prime series was not even available. Shortly after the Prime series was announced and you can get the the prime tower, prime center and prime bookshelves for a 5 channel setup for $1850 vs. $3,700 for the same in the ultra series.
Now... many will argue if the ultras are worth the cost upgrade and honestly I even question that myself. It's literally HALF PRICE to get the prime series. Guess what... I'm willing to wager for most but the absolute pickiest of persons will get 90% of the performance out of the prime series vs the ultras.
My advice is since your even asking and trying to mix/match for cost reasons. Just buy the prime series tower, prime center and prime bookshelf's. Your wallet will thank you for it and you won't even notice it.... after you pair that system w/ a pair of decent subwoofers.