need help choosing a mid bass sub for a diy

hi all, i'm wanting to build 2 mid bass subs in separate boxes. the hsu has a mbm-12 (12 inch mid bass sub) which has a frequecy range of 50hz-150hz. they claim it uses a " a very light cone, low inductance voice coil, and a strong magnet" which "yields extremely quick response with high efficiency".

i have a 2 channel sub amp (jbl s400 4 or 8 ohm switchable 2x200w) lying around so all i need is the woofers. my integra dhc 80.1's audyssey will calibrate the phase and such.

i need advice on a good mid bass sub which will perform well for home theater (room is 12x14 with an 8 foot ceiling). while the hsu mbm-12 uses a 12 inch woofer, would say an 8 or 6 inch sub give me quicker tighter bass due to its smaller size?

right now im using 2 velodyne cht 10's for mid bass and i feel that they're a touch flabby. they go down to 28 hz each. i would like the new diy subs to range 50-150 hz like the hsu mbm12, then use the my svs pb13 ultra to handle below that.

im looking for the quickest, cleanest, tightest possible bass. 6-8 inch subs are nice because the are small...but if a 10-12 inch will give me better punch and quickness thats fine.

just looking for some advice and suggestions!

thank you,
Look into VMPS subs. They have kits for diy, but they also sell fully assembled subs with and without built-in amps. The sound is adjustable to any room or system and the sound is deep, tight, dynamic, and musical. You would not be dissappointed. Plus the price would make it futile to diy.
You might want to chat with the guys at Rythmik, in Texas. IIRC, they started out in the DIY space and are nice, helpful folks who can probably offer you some good advice.

Good Luck