Yes, contact B&W USA in Boston (North Reading, Mass possibly). Check the website for the address, phone, or email.
This link might work...
http://www.bwspeakers.com/index.cfm/fuseaction/service.distributor&ccode=US&cname=United%20States%20of%20America
They likely have the original replacement woofers or at least a very close substitute. The 6.5 kevlar was on many other models. So was the black passive bass driver if the M804 have those also.
Those nice Matrix cabinets are worth saving.
.
This link might work...
http://www.bwspeakers.com/index.cfm/fuseaction/service.distributor&ccode=US&cname=United%20States%20of%20America
They likely have the original replacement woofers or at least a very close substitute. The 6.5 kevlar was on many other models. So was the black passive bass driver if the M804 have those also.
Those nice Matrix cabinets are worth saving.
.