I agree, an 8" sub or two can make an absolutely wonderful
woofer to round out a pair of bookshelf speakers or a pair of the "slim-line" towers with small mid-woofers, but it won't really do what most of us consider the duties of a subwoofer.
If your room is big, you won't pressurize it, but before I got my 15" subs, I was pretty content with the bass I had from a single 12" - compared to most 2.0 stereos, it sounded great, and at the time, my bar was pretty low - I had been using the built-in speakers in the TV!
If your room allows it, position the sub(s) close to your listening position - end "tables" or behind the sofa! That will do a lot to compensate for not pressurizing your room!
Even though they are only 10" and I seem to be contradicting myself, you might consider the (6" thin) 10" Dayton Audio SUB-1000L. It is the third sub listed in this well done (I respect Brent Butterworth's opinion) review. From a WAF standpoint, if these will fit under the sofa and the sofa has a skirt, these could go a few weeks before being noticed, much less detract from the decor!:
https://thewirecutter.com/reviews/best-budget-subwoofer/
It is $148 shipped:
https://www.parts-express.com/dayton-audio-sub-1000l-10-100-watt-low-profile-powered-subwoofer--300-639
It may not get as low, but I dare say a pair of 10" subs "close enough to bite you" will give you the thrill of a 12" sub across the room!
Edit - one nice thing about the SUB-1000L is I believe it would make a pretty good addition for most sub systems. IOW in two years if you buy a couple of potent 15" subs, you may still find you like a pair of these near!