It's more about how much output you want, I am using a Boss R1100M with a passive sub (4-ohm dual voice coil bridged into 8-ohm) connected to my Onkyo's LFE output. The Onkyo has about 65W/ch and I only listen to it at 50% or less, so rough safe estimate is 33W on the mains. To match sub power I estimate about 60W max output (since lows are more power demanding). double to 120W for inefficiency compensation. Old 350W PC power supply that has +12v@11A (~130w) is good enough, at max it gets quite loud where you have to yell to speak and things are shaking off the shelf from the bass.
have this setup for about 2 years now and listen to it daily, no problems.