What is DC offset on the AC power line?

This is fascinating stuff.  I came to this thread after reading about the  Burmester 948 Power Conditioner which is specifically designed to eliminate DC offset.   I live in an apartment building in a large city and I find that my system often sounds bad (anemic, glassy, cold, strained, two-dimensional, lack of power, no extension into the room) in the early evening (5pm to 8pm). 

I think davehrab may have hit the nail on the head with this:
" the real issue is the loss of efficiency your equipment experiences... 

The issue is that the loss of efficiency is not very audible like noisy RF can be ... your equipment just doesn't play with the authority or robustness it could or should"  

Hope this has not been mentioned earlier.  I had a hum in a Bryston 4ss2, called Bryston, they would get back, blah, blah, blah.  I went online and found a post where the OP suggested testing how tight the transformer was bolted to the chassis.  Not real tight, just good and snug.  I took the cover off the amp and used a wrech to see how tight the bolt was torqued for a reference.  It was not even finger tight, so I snugged the bolt down and the noise is gone.  Easy fix, no thanks to Bryston.