Usually the transformer hum is just that, I doubt if DR will make any difference, they can him a little, I have 9 135s and there is small hum, but this noise is only noticeable in a very quiet room if you stand near them, you can tighten the transformer mount at the bottom, if its bad its probably a new transformer that will provide the solution
Chance of hum is a higher risk with certain Naim models and is a trade off due to using usually big toroidal transformer PSUs which are more susceptible to DC offset in your mains. The DC offset causes the transformer to vibrate, I think usually at 50hz etc and so it's usually the DC offset in your mains that is really the culprit, but because cheap square transformers don't exhibit the issue people presume it's the toroidal PSU at fault.
The noise comes from the PSU itself and not via the speakers and is absolutely fine and harmless, it's usually low enough to only be heard in a quiet room or when up close to the kit.
You won't always get it as in the same batch of PSUs some will have none, some will have a bit and some will have a bit more.
You can reduce DC offset if you can find the culprit and it's in your house, it's sometimes things like a noisy fridge or hairdryer etc. Some people put in a separate clean spur for their HiFi directly from the consumer board, although not sure this will solve the issue, but it may make it better.
If the cause it outside your house, it may be harder to sort. A DC blocker filter should be able to do it, but they're not cheap (~£250-500), but then that's a whole other bag. Most standard mains filters should not be used with Naim and it vastly reduces their ability to pull mains in fast as they need it, which reduces their sound trait, i.e. slows them down and makes them sound a bit dull.
In my experience you want good well isolated and earthed mains with a very low impedance supply to each socket. This can reduce hum and usually offer increased dynamics with a lower noise floor. Any mains filtration or ‘processing’ usually compromises dynamics
Agree although I've read DC blockers with zero actual other filtering can be OK but I have only read that rather than tried it and don't have the issue, although I've heard it with a NAP 200 I had ages ago when I lived in an apartment.