look at RMS power ratings only, and you want the subs to be 80-100% of the RMS rating of the amplifier, generally. there are always exceptions, like audiobahn who outrageously overrate their products.. they have 1600 watt subs, but those really only need about 300-500 watts to reach peak output.
let me restate that. it sounds confusing after I raed it again. if the sub is rated to accept up to 100 watts RMS, the amp should be about 80 watts RMS. the RMS rating of an amp is the output of the amplifier, continuous, at peak output. the RMS power rating of a speaker is a thermal rating that tells you how much heat measured in watts, that the voice coils can take without melting. a speaker's power handling varies a lot based on the enclosure type, but as a rule of thumb, if you don't know any better, never give the sub more than it's RMS rated power handling.