Joe,
Shiva is a very efficiant driver. Adire Audio mentions it’s ability to run efficiantly at 45 watts and handle up to 650 watts. I own one and I can’t ask for more considering it’s small price tag. Even if it costs more, I’ll still be happy with it.

Dedicated sub amps are not that expensive (unless you want real high power). If you haven’t, check out this site: www.acoustic-visions.com/