First, I'd say that as long as you're not blowing the Monster's breaker, then you're not over-taxing it. It is rated for a max of 1800 watts.
Second, the units that you have protected are the most important. It would be nice it you could protect the sub also, but the TV and amps are more important IMO.
Assuming that you live in an area that has no significant power problems, then I'd simply buy a good surge protector for the sub to protect its amp.