Unfortunately, I don't believe the SX-1250 is compatible with a low output MC cartridge. You would have to use a step-up transformer to use a low output MC cart with the SX-1250. If it's a high output moving coil cartridge with 2.5 mV you would be OK.
Probably a no go for hooking up a powered sub woofer to your SX-1250.
Nando.