the UM's have attached grilles, which allows you to supposedly "seal" the front. they also have stainless terminals and the crossovers are sealed and attached on the back, which etduc can clarify, but i believe increases required mounting depth.
overkill imo unless you're boating on salt. the baskets and the rest of the motor structure are identical i'm pretty sure and i know the surface material on the midbass is the same carbon/glass fiber.
you are also no doubt paying a bit more for the certification they received from testing.
2003 X-2 MCX
Last edited by brucemac; 05-19-2010 at 06:23 PM.