The subwoofer has no transformer to block out the 60 cycle hum (all of Sunfire's amps have this problem, notice that Sony puts a transformer in their switching amp in the new 7100 receiver).

By putting a 50 hertz, 12 decibel per octave crossover between the signal output from the source and the subwoofer input it does indeed pull out most of the 60 cycle hum that is external to the sub what is left is from the subwoofer itself.

I live in an old house that was probably wired by the Three Stooges so we get a lot of issues with noise in the lines. I have had a lot of the house rewired over the years but not enough to eliminate all the issues.