Is digital contamination that big of a deal?

I had a fellow 'phile demo me last week the difference on his Maggie system with cable/internet box on and off.
There is a diff, nothing I would lose sleep over but its there.
I live in a big city with humungous power transmission lines about 3/4 of a mile away , From about 10AM to about 7pm there is a steady decline in my system, again not the end of the world but noticeable .
I've found MIT Z PC with its filter modules to help some.