The RMS rating of a sub is thermal power handling only. The box affects this only in regards to impedance rise. The rear loading affects the mechanical power handling of the sub which isn't related to the rated power.
As far as how much power you can run to a sub, you can run RMS continuously. That means a sine wave. That amounts to a continuous heating of the coil. The sub is rated to shed that amount of continuously applied heat. Music isn't continuous. Depending on the music, you can use an amp rated for considerably more than the sub rating without any problems. While I usually run considerably less than rated power because it's enough for my use, but I have run 2x rated power to a good sub on music, and know for a fact that it was seeing that power on peaks.