“When they are set up properly they really don't work that hard. If you can hear them you have them too loud/wrongly set up.”
@nubiann - That is a really good point. The other day, I shut down the amps to my main speakers in order to change some cables. When finished, I started to play music but had forgotten to turn the amps back on so just the subs were playing. Considering the size of the drivers and amps in those subs, it is a bit surprising how little output they contribute. However, if you turn them off when your system is playing music, they are certainly missed.

