Recommend looking at CSS-audio.com for quality drivers/cabinets. Consensus says go for sealed subwoofers for music (tight bass) and ported/passive radiators for movies (maximum bass output).
Suggest reading Floyd Toole's "Sound Reproduction" 3rd edition, the consummate audiophile primer for learning how loudspeakers/subwoofers behave in-room. In it he recommends 4 carefully placed subwoofers to reduce inherent in-room bass peaks/dips. I use 3 to good effect. As much as two smaller subwoofers work better than one larger one, adding one or two more is that much better. Most make the mistake of putting their subwoofer(s) in line with their loudspeakers. Think of a bathtub with 4 inches of water and moving your hand lengthwise back and forth. Waves will travel to the end where they bounce back until they hit the next wave where they will cancel, double up, or otherwise interfere with each other. The effect depends on frequency and location. Putting all your speakers in-line will just reinforce this uneven phenomena.
You shouldn't dismiss assembled subwoofers. Rythmik L12 is a compact servo powered unsealed subwoofer that only cost $1058/pair. SVS SB-1000 is another compact sealed subwoofer that start at $1000/pair. Both offer quality sound. Cheap subwoofers are notorious for weak, one note performance.
