Inexpensive but decent 8" subwoofers

I highly recommend the "slim" 8 inch subs by Monolith. They have 8,10 and 12 inch sizes you can slide under a sofa or wall mount. You can see the 12 inch (big) version mounted on my back wall if you look at my profile.

You also might consider the Audioengine subs. They have two models -- S6 and S8 (actually four models if you consider the wireless options for the above a separate model.)  I have an S8 and was quite pleased with it for the price, though it has now been replaced by a GE Supersub X (at 3X the price.)

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.