The sub draws less than a receiver would as it is only doing one thing so the receiver's input supply jumping around is no surprise considering what it is doing, audio and video. I would also suspect the amp section/power supply of the receiver is not as robust as the subs amp so it has to throttle the power intake to maximize use. While the subs amp section needs a minimum of 28 to provide the SPl you are demanding from it. I am sure if you turned up the volume on the subs amp the draw would go up as well.
