Weather or not a Naim box is going to hum does seem to be a bit hit-and-miss, and I can only assume this is the result of variation in hand assembled products. To help eliminate any external factors, you can try turning off all other appliances in your house, by unplugging them, or turning off whole circuits at the consumer unit. Turn things back on one by one to see if there are any individual culprits. These might include fridges, some light fittings, anything with a SMPS. There's a good chance this will not solve your problem, but it's worth checking anyway in case you have any rogue appliances.
Putting your HiFi on a dedicated circuit may also not solve your problem, but for many people it's a good sound quality upgrade at relatively low cost, so well worth doing in my opinion, even if the hum persists.