I just think that a 410 has more voice coils, and you here that in the tone. I don't know that I've ever thought of 212's as boxy though. They do typically sound more old school, with less mid range and articulation. When I hear "boxy" I immediately think of those Schroeder baffle cabs.
Less midrange? I don't personally hear that. Maybe it's just because I've got the old HT112....plenty of midrange there. I love my Berg 112's.