Are you using all 4 channels of the amp to run your subs or just the rear channels?
If you're using all 4 channels for the subs, your subs aren't the problem, it's the amp. Does the amp have adequate ventilation (not mounted upside down or stuck underneath something)? Are you using big enough gauge wiring? 4GA would be the minimum you'd want for that amp (assuming its ratings aren't grossly inflated). How about your ground? Is it the same gauge as the power wire, short as possible, and grounded in a good location for good metal to metal contact?
If you're using just the rear channels bridged to the subs, either 8ohm SVC or 4ohm DVC subs would be fine. There would be no audible difference, but it would probably be easier finding 4ohm DVC subs which also give you some wiring flexibility for future changes in your system.
