Revel M20 Woofer

No, the crossover was designed for that driver. You can't just drop in one that is "close".

in the not-that-distant-past Dynaudio charged me $300 for a TWELVE-INCH woofer, so for the Revel speakers, that seems to be unreasonably high. i'm no speaker designer but there should be a practical solution to this problem. but at the same time, a 1st-rate driver might cost more than $100.

Well, if youwanr some messed up speaker not worth anything. Go ahead andstick some other driver in there.
No one will notice except you.
And then selling them will be a problem, as anyone 'knowing will not pay half what you want for them, and it you LIE about it??
So cough up the money.
It's like you owned a Corvette, and decided an old Chevy Citation engine would fit, and be cheaper...