Subwoofer for "seamless" integration with Zu Druid


I've had a pair of Zu Druid Mk-iv's for about 3 months now. I'm really digging them, but miss the lower octaves. I don't know much about subwoofer design or manufacturers. Anything that would give me solid response down to 30hz (or even lower without sacrificing speed) but still be fast and tight.

What is the best design for integrating into speakers such as the Zu's (sealed, ported, with passive driver, etc.) and what brands/models should I be looking at? I'd like to spend no more than $1,000.

The REL's are reviewed well, and the Zu Method and Mini Method seem an obvious choice, but aren't manufactured anymore and are scarce on audiogon.
fargel
Sure -

I've got a Wavelength Brick DAC feeding a Musical Fidelity A308 integrated, DH Labs Silver Sonic BL-1 interconnects and DH Labs T-14 speaker cable -> Zu Druid Mk.iv (non-2008).

I'm coming from Vandersteen 2Ce's so I know this system will produce the bass I'm looking for if the speakers are capable. I listen to a lot of pop/rock and electronic music where a lot of it is obviously missing without the sub 40hz. :)
Take a look at the TBI subs for sale here on the Gone.
The seller has a pair but it looks like he will split them up and just sell one.
I had these awhile back and were the finest subs that I have heard, very fast with a nice organic sound.
Good Luck, Tish
PS The reason I sold them was because I purchased full range speakers that go down to 27hz. (Sonist Concerto 4's)
You need a REL Britania series B2,or B3 you can find these used at very nice prices..it is the only way to get seemless integration with your Druids. Check it out.