I can really recommend the Velodyne FSR or HGS series subs. Sealed box, front firing, adjustable corner points, a reasonnably steep slope to cut the subs top end off (12 DB per octave increasing to 48 DB per octave after the first octave)and they are QUICK. I've had both the HGS & FSR subs and I feel the older FSR series sounds better for a given size cone. The high pass section is quite transparent and lets the benefits of bi-amping come thru, however, your corner point is limited (80 or 100 HZ)and the slope shallow (6 DB per octave). The good news is that the FSR series can be had cheaply on the used or N.O.S. market (about $800 for an FSR-12, for example). If you want a steeper slope to cut off the bottom of the Merlins, you can go to an outboard electronic x-over from Marchand (highly recommended)and bypass the internal x-over in the Velodyne (called sub woofer direct, or something like that). These guys are as quick as the RELs (an excellent series of subs by the way)and can keep up with my Apogees. Good luck.