It depends on what driver, and what you are asking that driver to do. Trying to push an Eminence NSW-6021 is a lot different than the Dayton UM-18.
I'm not suggesting that you actually need 5000w to drive that NSW, but if you want that to perform down low, you have to pay the piper with power to push that motor.
*shrugs
Even Nick from SI suggests having a capable Amp to fully power his Drivers. The SQL15s I'm building Subs with, he actually says I should have 2000w available for them!