I may have another M4 crossover around here...
The reason I want another M4 cabinet and matching tweeter is that they are the right size for either my bench or music recording center (synth and reel to reel) and I've a QX-646 for a 4-channel monitor with my Teac 2340. I have three other small pairs that I can make a quad set with.
The MW 6510 is from 11t at least, that's what is stamped on the back. I have a search for it on eBay but the stock MW 6502 is a lot more common and used over several models.
Not to mention it has a lot more guts than a Baby Advent, which I don't like much because that woofer can't take much bass and sounds bad in a cabinet that is close to the same size.
Maybe I got a bad pair, they were Baby IIs and Recoton/Jensen era but I've also been told the 6510 was a 'big upgrade'.