Outlaw was quite right to steer you away from plugging the LFM-2 (or any sub or amp) into a receiver or processor's accessory outlet - those outlets are not intended for loads larger than a DVD player or VCR.

It is fine to leave your sub on all the time, but if you want to you can set the power switch on the sub's plate amp to its middle position ("auto"). In this position, it will detect when a signal is being sent to the sub and turn on, then after the sub goes for several minutes without getting a signal (typically around 15 to 30 minutes, to be sure it's not just a quiet passage) the sub will shut off (red light instead of green).