Heard the B&W N804d3s ...

rlb61, 

I cannot say if the bass above 25 or 30 Hz is noticeably better represented on the F12 than the L12's, as I was going to try them but went a different direction. They are both well regarded by users.  I did try two smaller sealed SVS subs in a room about twice the size of your listening space to match with some Monitor Audio stand mounts, and sent them back as they just didn't have the sound quality I was looking for.  I replaced with B&W PV1D and and extremely happy with that. I own a PSB sub in another system that lists for more than the speakers and find it very musical. YMMV.

The L12 may do the trick with the 804's, but there are good reasons to buy a high quality sub beyond it's low frequency capability, not the least of which being speed, tone, and integration with your speaker's bass drivers. No matter what, I recommend a decent sound level meter and some analysis software to help you tune your sub to your speakers and your room.  Enjoy.

kn

UPDATE ... So, I received the L12 yesterday and have integrated it into my system with a cross-over at about 55Hz. Integration was seamless and quite easy. The L12 has made a huge difference in that: (a) it gives me that extra bit of articulate, bottom "oomph" for which I have been looking without sounding boomy; and (b) mids and highs have more clarity and presence, perhaps due to relieving the cross-overs from digging down deep. This is my first experience with a sealed subwoofer and it has really improved my system and listening experience tremendously ... plus, it has prevented me from spending $9K on new speakers, just to get that little "extra" low end which the L12 provides in abundance.

@rlb61 

Congratulations, enjoy, sounds like the Rhythmik is doing a great job.

Cheers,

Scott

Thanks, Scott.

Success!