For folks using the North America power grid: Is DC offset usually a big problem. I've never heard transformer hum from any of my gear...
According to Rod Elliott
"...
There are many misconceptions about the use of DC blockers, with some self-proclaimed 'experts' insisting that they are snake oil, because DC can't get through the mains distribution transformers. This shows a complete lack of understanding of mains distribution, how offset can be created, and just how little DC offset is necessary to cause problems with larger toroidal transformers. This is
not something that I just dreamed up - I've been able to measure DC offset, and as described above, a hair dryer that uses a diode for half power can be enough to cause a large toroidal transformer to blow the mains fuse
or even the switchboard circuit breaker!
Likewise, I can't bear to hear/ read
actual snake oil vendors claiming that you'll get "cleaner highs", "more authority in the bass" or any of the other stupid things you may read elsewhere. The purpose is to stop transformers from growling (usually at no or light load). There is no magic, and it doesn't improve anything other than reduce the acoustic noise from the transformer(s). Anyone who claims otherwise is probably lying.
You need to add a DC blocker
only if you can hear power transformers growling intermittently when the amps are running but with no (or very low) power output. Adding one will
not 'improve' the sound, it's purely a preventative measure that ensures that mains DC offsets don't cause audible (mechanical) noise. This isn't a panacea, but if you do have issues then it's a reasonably cheap (and effective) way to minimise the transformer noise. There may be cases where the DC blocker may not be effective (transformers can also growl if the mains voltage is higher than normal).
..."