I have been fighting the hum for a while now. Worst offender was my NAP300DR PSU to which i attached an Audiolab DC blocker three months ago, which helped but did not cure. Most recently i realised i was getting hum not only from the NAP300 PSU but also some from an XPS DR PSU and most surprisingly my 3 year old Rega Aria phono stage was now the loudest!
So i have now purchased a high-power DC blocker from ATL Audio that has two UK socket outlets and this now feeds my two power distribution strips (and removed the Audiolab device). The silence is refreshing! I can still feel a slight non-audible vibration in the case of the Rega but the two Naim PSUs are still at last. Listening to music i cannot yet discern any reduction in audio quality by running all the power through the ATL DC blocker. HIGH-POWER DC Blocker Trap Filter, 2 AC Outlets – ATL Audio Ltd.
Hi @kwhitmarsh the ATL DC Offset circuit is more less identical to my own filter design, mine does not include the 22ohm resister, but I suspect that will change in the near future.
One thing to keep in mind is transformers do want to vibrate & it can be audible to some extent.
My Naim transformers all hum, but are only just audible in night time silence & within 1m distance.
Finally my hum levels reduced even further a year or so ago when the local national grid voltage was reduced, mine went from 248/252v to 238/242v.
something strange is that the hum was not constant, but only during some hours of the day and of the night. it was getting me mad as I could not hear the music well during the night.
I bought IFI dc blocker, solved 90%, strongly reducing the hum, now to hear it I have to get very close to nap 250. moreover both hicap dr and nap 250 were humming, I put the IFI only at nap 250 and also the hicap is not humming any more.
I cannot say it impacted the sound and is very small, smaller than other DC filters I saw.
anyone thought about possibility to bring devices to assitance? at which level of hum a device should be brought to assistance?
This is indicative of some form of pollution, probably DC, and if the iFi device helped, it most likely is. But it could be just noise from nearby loads, or issues on your local supply.
I almost didn’t dare post my experiences with transformer hum- SN3 and HCDR!
But- I have become irritated by variable transformer hum from both these components and after following all the advice on the forum concluded it must be dc offset.
I purchased an ifi dc blocker and it made absolutely no difference at all. I concluded that the component was insufficient rather than questioning the diagnosis so it went back for a refund
I then saw a isol 8 axis dc blocker for sale at less than half retail so bought this.
This too has made no difference to the hum- so I owe ifi an apology.
However- the sq seems to have improved- more width depth and height to the soundstage and bass tighter. I may be imagining this and trying to justify the purchase of a new box- who knows?
The hum continues unabated so I think I will just have to cope with it!
I assume you have been through everything in the following thread?
Sadly yes. I have concluded that I just have been unlucky with a humming transformer.
It’s the same when I tried it in a different plug socket in a different room in my house and also in a different house.
In fact I must thank you because I have just realised that you wrote the authoritative guide on this subject!
It is a subject that comes up so often, and no-one was putting all the combinations of responses together. I’m just glad to help people. I still have some hum, but not enough to worry about
Try the above, a Excelsior DC Blocker MKII. I had hum problems with a XPS DR and a 555PS DR, until I used one of these. My listening position was close to the power supplies and the hum was noticeable, unless played at thrash metal levels. After fitting, I could play at low level and not notice any hum.
P,S. If anyone has a strange fetish or obsession about placing your ear next to the power supply, then you will most probably still hear a very low level hum emanating from the power supply still. 
I believe MCRU offer a 30 day money back guarantee.
Just ask them how many Naim or other manufacturers owners have returned one. Be prepared for laughter at the other end of the phone
Many thanks - I’ve bought a few bits and pieces from them and always been really impressed. 30 day money back is the deal!
I think the exception on the 30 day, is it is made to order. Just check with them before purchasing. They are very helpful and straight talking Yorkshire people i.e. no bull. 
Were the two boxes close together, on the same rack? It’s possible that the mechanical vibrations from the 250 were transferred to the Hicap through the shelves and that set it off.
When I had a 250 the rack was right next to the sofa, and if it started to hum I could silence it by just resting a finger on top of the case for a few seconds.
I had the exact same experience. The isol8 axis gave a noticeable lift in SQ, particularly refinement. What sorted him was better power cables (Chord Shawline). The isol8 is great value though. I found the IFI killed the dynamics a bit.
Really interesting- tbh I didn’t even have a proper listen to the ifi as it did nothing for the hum. I have read elsewhere people make similar observations about it though.
The other advantage of the isol 8 is that it is one cable to the wall plug so disconnecting all 5 components is very quick and easy- eg if we are away etc.
Just last night I saw a very positive review of the Inakustik AC-4500 Reference Power Station on YouTube. It filters DC and looks the business. Only six sockets though. I use an Audioquest Niagara 1200 thats decent but doesn’t do anything for transformer hum.
I am trying the IsoTek EVO3 Synchro Uni 10A DC blocker on my 500 system. It reduces the hum to a quite acceptable level (but not completely) with positive effect on SQ (better bass). It should stay
there is an interesting DC blocker coming from Bulgaria at a good price, some people I know found it works well and for sure is not expensive.