If you can afford the Seaton, go for it, I think it would be a definite upgrade over the PB13. As for your rears, the Bose cubes probably aren't totally awful as surrounds but even in such a minor role I would prefer better speakers. I would go for monopole speakers, I think even some inexpensive direct radiating speakers would be a marked improvement. I would also note that its recommended to have your center speaker match your left and right fronts. You might think about replacing the Richter with JBL LS center. It wouldn't be cheap, but your front left right speakers are terrific and I think they deserve a matching center.