It seems you have a lot of experience with acoustic panels allready.
You probably know this product allready, but if not have a look:
https://www.gikacoustics.com/product...bass-trap-t40/
I believe it would take at least 8-10 panels in your room.
But these should be very efficient, as they can be custom tuned to your specific frequency.
Another approach would be to ask a studio-designer to calculate a helmholtz-resonator for you. You can do the expensive part of building it yourself...
When finished, the studio-designer should visit for a last fine-tuning...
Both of these approaches will cost 2000-3000$, but if you can spend 1500$ on woofers this should be considered.