I've noticed that a lot of lower priced and even mid priced active monitors have quite a bit of hum, regardless of the grounding scheme.
It's most likely due to poorly filtered power supplies. Many of them don't even shield the preamp section, so there's probably some RF and power transformer bleed in there as well.
Depending on what kind of monitors you have, there may be nothing you can do short of redesigning the amplifier section.
Thomas