Inexpensive but decent 8" subwoofers

You may wish to look at Totem Kin subwoofers. Their ultra-compact 8" sub sounds fantastic, is extremely well made, reasonably priced, and looks great too.

Klipsch sw308 or newer model equivalent if you can find one. Front firing woof and two side firing passive radiators deliver extra bang for the size and also makes it easier to integrate well. I use one with a pair of KEF ls50 meta in a smaller room and the combo checks all the boxes.

@resveratrol3 note that any smaller sub no matter how good may not be a good match to some speakers and/or room. If speakers are fuller range and not high pass filtered you probably need a larger sub to properly fill in the missing lowest octave. Also the larger the room the larger any sub up to the task properly will probably need to be. A smaller sub running with more full range speakers to start will be a challenge if even possible to blend in properly without merely delivering a big bass bump in the frequency range already covered. That could result in fat muddy bass that also obscures detail in the midrange which is not a good thing.

MK 70B, push pull design. I'm running 2 upfront & 1-MK125 for my surround.. Check all the pos. feedbacks on this brand.

Jamo has a couple of vertical subs that are pretty stylish and inexpensive.

Never heard them though.

Hope you find something you like!