I know a sub is really designed as a *sub* woofer. I would like to find just a woofer, that is very tight and musical, maybe 8" sealed? Does not need to do 20hz, not helpful In an apartment. I have an HSU stf1 that is ok, looking for a change.
I will echo kesando as to why you want to change from the STF-1 which is a fine sub for smaller places. If you just have the bug, you could look at the STF-2 or SVS as they have a couple models at $499 shipped.
Sealed box for sure, I'd go at least 10", a small 12 is probably better. Front not down-firing if you have downstairs neighbors. The apartment is hard, which is why I'm running full-range towers and no sub at the moment.