• Sound Quality - I'd expect it to blend flawlessly with a flat speaker crossed at least up to 80 hz (i.e. flat to a reasonable headroom above 80hz).
• Deep bass - For certain music and movies this is quite important. I'd say +-2 dB down to 18Hz is great, but 20Hz would be good. For many, 25Hz would suffice, but many others, including myself, have to be swayed to upgrade from another (likely decent) woofer.
• Output capabilities - This is tied to deep bass for me. I often have no limiting factor on volume in my system. However, with my current HSU3.3 for deep bass heavy music or movies, my sub is my limiting factor. For movies this is more of a raw output issue. For music (e.g. 16.5 hz organ pedal), port noise limits my output. This is still rarely an issue (and often not an issue at all), which is why I rank it #3.
• Size - A smaller size can make placement easier, which helps sound quality. But since a bigger size tends to have better quality and extension, I put this low on the list.
• Finish quality - I might consider paying more for a piano finish option.