Well, I wouldn't use BL to tell control as it's really more indicative of the motor strength. Just using the printed value won't tell you anything about what happens when the sub starts moving. It's the BL vs x curve that you want. Some fall off incredibly fast while only moving a couple mm. Some will have a wider curve keeping control with much greater excursion. Subs with XBL^2 technology for example. They're BL is about half that of a Digital Designs sub, but the Xmax is several times greater. This means the sound will be much more consistant with increasing excursion, be it from lower frequency and/or more excursion.

The tightness also has a good deal to do with the installation quality. This also goes with the setup in the room. Just saying that stereo or mono is better is not really the best way to do it. The subs in question are already built, so everything has to do with the placement and setup in the room. Take some time and try them out in the different configurations. Spend the whole afternoon if you want. There's no rush and you really can't get a good feel for it with 10 seconds of listening.