Good passive subwoofer?

I have an unused Adcom GFA-555 amplifier rated at 600 watts bridged mono into 8 ohms. I am thinking about using it to power a passive sub. Does that sound like a good idea? It would be mostly for use with movies, as I am happy with the full-range bass in my KEF 104.2's for 2 channel music. Any suggestions for a passive sub? The signal path would be from the sub pre-out on my pre/pro or receiver (which I haven't chosen yet). Thanks!