I think the most overlooked objective when shopping for a sub is getting the right design for your listening room. You first need to decide whether you want a front-firing, downward firing, bipolar or other. Then you should shop for the best sub in that catagory.
In my smaller listening room, front-firing subs just overpower the room. I need a downward firing sub (HSU) to spread the bass out more evenly. I found this out by trial and error. I started with a good sub (an M&K V-125), which sounded good in the store, but sounded less than optimal in my listening room. My room had uneven bass responce that didn't sound very good this way.
In my smaller listening room, front-firing subs just overpower the room. I need a downward firing sub (HSU) to spread the bass out more evenly. I found this out by trial and error. I started with a good sub (an M&K V-125), which sounded good in the store, but sounded less than optimal in my listening room. My room had uneven bass responce that didn't sound very good this way.