Well, it depends on how technical you want to get.
For one, it certainly is a waste of energy albeit a relatively small one.
Second, any time electrons flow through an electronic component, that component's lifetime is reduced. Now how much it gets reduced just sitting there idling can be calculated if one has the component loading model. I would venture to say that at idle, the effect on lifetime is negligibly small.
I believe there may be a remote input on that sub that can get triggered from your amp but don't quote me on that.
