You are going to want a sealed subwoofer and there a variety of very nice ones or have you looked at the bass panels from mageneplanat? What is your budget?

THe Sunfire XTeq 8 is $1500, has more power than you could ever need and has a dynamic equalizer to ensure seamless integration with you maggies. It is also sealed with an extremely refined sound. I have a Sunfire Tru SuperJunior which is model prior and it is an awesome sub.

The Sunfire HRS 8-12 are less at $850- $1050 but lack the dynamic EQ and are harder to integrate but are lovely, very refined, highly detailed subs. I have an HRS 12 in my home theater.

The Sunfire XTeq's have an XLR input and the HRSs have speaker level inputs.

Rel has the T/9i at $1299 which is a great sub. The only thing I find annoying about Rel is they use an obscure (albeit high quality) connection type for the best possible connection which is Neutrik Speakon. The irritating part is the cable is like $450. Otherwise you are stuck using an RCA.

SVS also makes nice subs. The SB2000 is a great value at $699 but won't be as refined as the Sunfire's or the Rel's. The SB13-Ultra is $1599, a hair above your budget and is really nice sub and in the same league as the Rel and Sunfire's. It has XLR and RCA inputs.

There are tons of other options from other companies. These are the ones I know and like.