Sub for Acoustats 2+2

Most people here are going to recommend a REL sub. It integrates well with everything.

It all depends. Personally with my Acoustats and the music
I listen to, I have never needed a subwoofer. However, I
have tried a friend's Sunfire Sub. It integrated well
with the system and played rock and roll with authority.

I'm listening to my 2+2's with a pair of Muse 22's. Sounds very good (to me). Rock solid and good looking (like me, not...) Muse 18's have a card available for the speed issue as the Acoustats are relatively fast, and a slow sub will weird out the sound. IMHO.

With my Acoustat 2+2 I use Velodyne's sub, ok it's not an ULD
original... I don't like the connectors I change it! I don't like the internal wiring I change it with Vampire! I don't like the alimentation I double it!
Now it's OK and I don't feel the sub I just hear a perfect match where the music take the place without the feeling of the speaker or the sub box.

Sic

I owned 1+1s a long time ago. You definitely need a "fast" musical sub. The REL used to be the standard, but the ACI Titan II LE has really passed it by. At $1100 direct it is the new reference in musical subs. Has about as much output as the $2500 REL. Very fast and defined. Built like a tank with a gorgeous furniture grade cabinet. I have done extensive comparison with the REL, the Titan II is clearly better at less cost. Got mucho applause in the latest issues of Positive Feedback, Bound for Sound, and Soundstage.

An absolutely great sub. Has far exceeded my expectations. After over a year of trying to find a sub that wouldn't get in the way of the music this is definitely the one. Lots of flexibility with the filters/controls/inputs. Everyone comments on how well this beatie keeps the beat. This is a serious high-end sub at a midfi price.