I installed a Kef Reference 8B for a client recently... I was worried it wasn't going to hit low or have enough power for a large room... I was wrong, not sure how they make two 8 inch woofers move that kind of air but holy smokes that thing is legit, it can go very low and very loud if you want with authority.. It is $8k retail but wow, its awesome and looks beautiful and compact, its magic how much sound that little sub can put out
In the more affordable category the old Sunfire HRS 10 or HRS 12 is another great compact sub that I have installed dozens of and they hit hard, deep, tight, and clean... 10" is 11.5" cube, the 12" is 13.5" cube, now that Sunfire is gone, they are still making them but under the Speakercraft brand, one of the best subs for the money and the very compact size