It's likely that the S8 has a more weight in it's cabinet, in part because it's nearly 10 inches taller than the M80, but also a couple of inches deeper as well. It may also have more bracing to go with the cabinet as well. Also the S8 has 5 x 7" drivers vs 2 x 6.5" and 2 x 5.25" on the M80, so there could be some added weight there on the magnets...the M80s are hefty, but certainly not nearly as much as the S8s. Sometimes, and I'm not saying it's the case here necessarily, extra bracing is just...extra bracing.
