21" woofers not sounding good

Since you are more concerned with the fidelity and less with the level, and described that two ported 18" were enough or almost too much level wise. 

I am running two cheap 12" woofers in very compact u-frames, tucked inside a closet, powered by a bridged crown xls 2500 (corrective boost applied with dsp) . The room is admittedly small, around 12 sqm, concrete walls, large, full bookshelves up to ceiling height.

This setup gives me very pleasant, strong sub down to 30 Hz. It is relatively tactile, although it doesn't go insanely loud. The bass is also very articulate, dry and nlt boomy. If the bass instruments are well recorded, the pitch of the individual notes is easily perceived. Opposite of the  muddy one-note experience.

It's very easy and economical to experiment with. If it doesn't work out, you don't waste much wood, and can still build something else.

If the room permits, several 15", 18" or even 21" woofers should give you good results!

Look for drivers with fairly low Fs (25-30 Hz range) combined with low(isch) Qts-Qes values (roughly between 0,25-0.33) and BL as high as possible (preferably above 20).

How did you process the signal for those OBs? 

Nobody will be surprised by that revelation…

Small diaphragm drivers trying to do mega low output always run into the same issues: excursion related non-linearity and mechanical noise.

A large diaphragm driver like an 21” or 24” for the same displacement has to travel significantly less distance (and usually have a higher motor force for the volume of air they’re moving to boot).

The limiting factor for what you’re talking about with sealed cabinets is that the corrective EQ comes at the expense of headroom and power. The 2150 box I built at full output would draw about 50-60W and should achieve 115dB @ 1m… HOWEVER it would require an amplifier capable of about 3.2-3.8kW into 8R and steadily rises to that level from about 60Hz downwards to achieve that when you consider the gain of the corrective EQ.

I stuck a 20Hz HPF on mine because I have no interest in frequencies that do nothing but piss my neighbours off, piss me off with rattling furniture and fundamentally waste power from my amplifier’s rails. Run from a modest amplifier (600-1,200W @ 8R) it would be more than capable of providing sub for a large-ish house party. I’m running it on a channel of my Cloud CX-A6 and that is capable of more output than is compatible with my rental agreement 😂

The Beyma 21 has an EBP of 83,3, so perhaps try playing around with the volume of the sealed chamber to get something closer to what you want to achieve.