The transformer could be burned out by lengthy operation at that power. What you would "blow" would more likely be your hearing in that case. The M80s are slightly above average in sensitivity and use about 1 watt for a comfortably loud average level. Brief peaks use a lot more, but are unlikely to need more than about 100 watts, which your 875 can easily handle.

Which ground loop isolator did you get.

http://www.jaycar.com.au/productView.asp?ID=AA3084

I bought this one.

try unplugging the sub from the wall outlet.