From what i can gather reading through various posts and other sources online the 309 is not all that great of sub so if you want to sell it you will definitely get something better.
The B&W you mention is around £499 online and has a 200w amp with a 10" driver, compare that to what BK offer with the monolith 300w amp (500w with the mono+) with a 12" driver and many happy owners here and elsewhere for around the same price, to me that is a no brainer.
I have no experience with the B&W sub at all but i cant imagine it out performing a monolith or even an xxls400 with those inferior specs for more money.
Take all this with a pinch of salt though aslong as you're happy