Need advice on subwoofer


I've been upgrading my circa 20yr old system since visiting a B&W shop in Beijing (of all places). Discovered that I was not deaf after all. Being British (tho living in US), B&W had some appeal. Also wife factor important in mycase & B&W's look cool. Now have B&W 804s front, B&W 805s rear and 1 804 as center. (Fedex lost one 804 in shipment hence the 'spare' 804). Denon 3805 AVR. Music tastes vary, Blues, Rock & sometimes classical. Also hook up Denon for DVD's, TV etc, but main love is music. Room about 14' x 18' high ceiling. Extending to kitchen/bfast of 14'x15'. Now looking for subwoofer to complete system. Although I like the 'brightness' of the B&W's, maybe a sub may add some balance at the bottom end. Been looking at Axoim EP500, Paradigm Seismic 10 or 12, and Velodyne DD's. Suggestions welcomed! Also, strange request, anyone tried putting an 804 on its side to use as a center?
dnallob10001
Your Nautilus speakers are very refined, so you do not want a cheap sub to muddy up the bass. This is one reason I recommended the REL Stsdium or better.

I have a REL Storm III that I used for 2 channel until I got my latest speakers. In the past the Storm always made the room sound better. My new speakers have reached the point where that is no longer the case.
Another vote for Rel. I'm using a pair of Strata III's
and they are excellent. Rel subwoofers have a highly
adaptable crossover network with provisions for both
high and low level inputs simultaneously. I have mine
set up on low level input with my 5.1 home theater sound
system and on high level with my Magnepan 1.6QR's for
two channel stereo. Rel subwoofers are easy to integrate
with many systems including challenging matches such as
Magnepans.
The best place for specs is the REL website.
The REL website is hard for some to find because it is not a .com

Click here to find the REL website ....

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thanks for all the advice thus far. REL looks wonderful, but so too are the prices on the SumikoAudio US dealer site. I may have refound my hearing, but the screech of the dollars leaving the wallet (I'm Scottish) might not be totally countered by the bass notes from the REL! I had an 'open box' Velodyne 15" DD from Tweeter last night..I had gone looking for a 12", but they didnt have that & the 15" DD had 200$ off. 15" vs 12" to me is 225 vs 144, so 56% larger. But there was a reason for the open box...it was DOA & missing the handheld. Still, it looked cool and helped establish a "this is what a woofer looks like" rapport with the other half...(this is important) So I'm still looking.

Anyone with a comment on running a Nautilus 804 on its side as a center?