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.
I own the DruidV and a pair of Zu Undertone subwoofers. It is an excellent combination. The integration of the subwoofers is subtle but very noticeable when they are turned off. It gets the sound down into the lower end. Talk with the folks at Zu.
You should definitely call Zu. They may have a few remaining Methods, etc available.
I know nothing about your speakers, but subs with Maggies are notoriously difficult to blend. The Gallo TR 3s I got are marvelous with my 1.7s...might be worth a look for you.
Over your budget, but Zu's Undertone is excellent with my Druid V speakers. It blends in perfectly.

Talk to Sean, he might have some b-stocks for a good price.
I have a pair of MJ Acoustics Ref 100 which work well with my Zu Druids. I find them to be quit musical and to blend in well, as do the REL Strata. It seems most high powered subs do not integrate well, so lower powered musical subs worked best, for me and my Zu Druid Speakers. I am looking forward to hearing the Zu Undertones though.