I thought that was you who previously owned the PB-10 and -12. Didn't you replace them with an HSU? It is going to be somewhat difficult to get both extension and musicality at $400. Seth already said it - if you weren't happy with the SVSs, the Infinity isn't likely to do a better job than either of them. I found the CSX-15 to be fairly musical, but it doesn't really have authority down low. I'd say STF-2 or VTF-2 from HSU if you haven't already done so. I used to have the VTF-2 and the PB-10. I kept the PB-10 for extension and output, but the VTF-2 was a bit better with music. That was the 10" version of the VTF-2 though, the current model is a 12" (and $500). This might be the best trade off as you can configure it for output or extension depending on your needs; I left mine in max extension all the time.
A sealed sub should get you where you want to be musically, but unless you start looking at a much higher price range, it is going to be difficult to also get good extension.