This is what I'm thinking. You owned the Rythmik F12, which is considered an excellent subwoofer already.
So if you are going to "replace" it, I think you might look for something different, not another equivalent smaller size sub.
Another point you might discuss is the sealed vs ported design. The myth that sealed subs are somehow "tighter" or more "musical" is really just a myth. Look at the bass from the Revel Salon2. It is ported, yet tight and musical like any sealed system. I think in some cases "tighter" and "musical" might be confused with "less bass"; turn down the bass and all of the sudden it's tighter and more musical.