What is DC offset on the AC power line?

DC off set is a much bigger problem than most audiophile's realize ... the humming transformers are merely the symptom ... the real deliberating disease is the loss of transformer efficiency 

DC off set is caused by the half wave bridge rectifiers used in all switching power supplies that draw current in uneven pulses .. this drives the sine wave off it's proper zero degree crossing point when the phase turns from positive to negative in the cycle

The humming in the transformers is caused by the transformer drawing an extra cycle and the plates snapping  back ... but the real issue is the loss of efficiency your equipment experiences  

Just about every appliance in your home including that flat screen TV ... cable box all your computer devices and some of your audio equipment uses switching power supplies and cause this problems with the harmonics they create  

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  

I have a Emotiva CMX2... which also has some  common mode filtering ... on my wife's plasma  

I never A/B-ed it just bought it and installed it when I bought her a new 60 plasma .. along with the Oyaide R1 outlet and matching carbon fiber plate .... happy wife happy life 

I also use two PS Audio Humbuster III which I wouldn't be without in my 2 channel set up ... they had a profound impact  

Just today someone on Audio Circle put a used  Emotiva  CMX2 up for sale for $70 

I would have posted the link or sent it to the OP but the last time I tried to be helpful I was blocked  

I've seen schematics on the Bryston web site of their big amps and they definitely implement a DC off set circuit ... it's two  electrolytic caps in series by passed by a diode bridge ... the caps center the wave and the diodes prevent damage from reverse biasing ..

Bryston also uses a ground blocking circuit on the safety/ground wire with a diode bridge that allows a fault to be conducted to the ground rod but blocks any voltage being conducted on the ground wire in the opposite direction towards the amp

 Not sure who else or if any employ these circuits

Stringreen ... I'm going to go out on a limb here but I think your amp's protection is against DC (direct current ) riding on the the line and most likely the analog ICs ... the amp shuts down when it senses the DC current so it is not pass onto the speakers were it can damage the drivers ... DC offs set does not harm the speakers only reduces components efficiency

gdhal     if you could develop a perfect world scenario with no switching power supplies and half wave bridge rectifiers there would be no DC off set to defeat and no use for a blocker

But the blocker can serve another important role if and when not defeating the off set

I think the worst offenders are the commercial and industrial users that use 3 phase power ... they create a lot of harmonics that get back on the line and pollute the grid that you draw your power from

The  power your components draw from the grid  is loaded with these line harmonics that do the same thing in reducing transformer efficiency and you have the same issues ... reduced dynamics and head room

So even if you can evade the DC off set scenario some how ... you still have a big issue with the 3 phase users distorting the line and here is where a blocker like the Humbuster can be beneficial    Enter your text ...