if you know the application its helpful when picking the sub.
If its HR for Movies and Home theatre...the booming aircraft and helicop's...larger cones, ported etc. 12", 15" work very nice.
With music, 8" and 10" has made some happy.
Blue Sky has the sealed cabinet, tightening up the low end and seem to have put more into the "sub/sat" design than many others. Some good articles explaining why the sealed cabinet (acoustic suspension vs ported).
I know the little RTA stuff I played with last year showed most my 6", like your JBL's, fell off at 150hz range..I know the specs say 50hz or 45hz but thats in a anechoic chamber, as I understand it.
So we're not hearing near that low freq, in a typical HR room I'd suspect?
and the RTA and DB showed this in my case. the yorks, ram's, jbls, whatever none of them went much below 100-150hz per the meter, if i recall.
the sub when added, brought the low end up, even though I couldn't really hear it, the Calibration mic picked it up.
the room....the room and speakers, together like peanut butter and jelly...
and then there's Barefoot monitors...