Best subwoofer for Linn AV system


I've been reluctant to buy a sub, living in a condo.......but the upgrade bug is going strong and I'm considering buying one.

My system consists of Linn AV5103 pre/pro, 5xAV5105 amps (soon to be activ for the front) which consists of AV 5140's and an AV 5120 center, Snell K.5 surrounds. I'm inclined to go with the Linn sub (AV 5150), but it is $5000.... I'm wondering what experience others with similar systems have had with other subs and how it's worked out.

Any help would be greatly appreciated!!
kennyt
Don't think you'll do significantly better than Linn's own sub (expensive as it may be...it's designed to work with the Linn's). The old axium..."if it ain't broke, don't fix it". or..."if the team works together, don't change the line up!"...and there you are.
I too have the 5140's and 5120's. I tried the Linn sub. It just would'nt play loud enough. I ended up with a Velodyne HGS-18. It out performed two Linn subs at a fraction of the cost.
Hey Curt, I've considered an HGS 15, or possibly an 18, just how loud do you play your system?? Do you think the HGS 18 is needed, or is the 15 enough??

What kind of difficulty did you have integrating it into your system?