Matt, welcome. Yes, it may be that you have the sub output level on your receiver set too low to keep the sub amp "awake". Set the receiver sub trim higher and to compensate for this set the level control on the sub itself lower. If it worked well before, however, it's possible that the sub amp has a defect which has now appeared; if so, you should contact Axiom directly.

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Enjoy the music, not the equipment.