The SX line is similar to the SSD line with the flatwind coil option (same cone, spiders, and coil when the flatwind option is added in). It uses 1 less magnet, but has similar motor strength. The BL in standard form can take more physical abuse than the SX can due to the increase number of spiders in the pack. Then it can be upgraded with options from there.
SXs are rated at 1000W RMS. I would never be able to make them take 2500W on a daily basis... SPL yes, but daily they simple dont have the thermal capacity (or mechanical) to do so. We really dont recommend our top line BTL for more than 2500W daily for most applications as well (although everyone seems to push them with more than that).