Have you actually tried bass shakers? They are NOTHING like real bass. Not nearly as satisfying. I would go for two of the Dayton 12" subwoofers. And you can get a tighter/more accurate perceived sound from the Daytons by simply adding some more internal acoustic stuffing. This results in lowering effective tuning, and at the same time, reducing the likely peaky output from the port. The combined effect results in 'tighter' bass, or so how most would probably describe the sound. 2 of these are optimal as your small satellite speakers need to be crossed relatively high. When this is the case, you need to have a subwoofer near each main Left and Right channel to prevent localization and to result in better seamless integration over-all.
BTW, in the future, replace all of those satellites with Behringer B2030P monitors ($130/pair) and you have a VAST sound quality improvement. Despite the low cost, the Behringer monitors have no comparable competitor near their price range vs. their actual 3rd party verified performance. Also, build quality is far higher than anything else anywhere near it's price range. 3 pairs ($390) and you are set.Then maybe down the line upgrade your reciever with one that has much higher output power.
-Chris