I’m confused. Why would you buy a super tweaky 300B 8 wpc amp? After all your prior difficulties ? You buy an amp that is bound to be problematic. If you want to do tubes there are several well made great sounding amps that are almost problem free. Ayon Audio, Octave Audio, Audio Research… BTW have you checked the Bias on the tubes?? Good luck…
Thanks again all. This is actually really helpful in clarifying where I think we are.
Given that Zu speakers have no crossover but use a high pass filter to activate a supertweeter above 10Khz, can we put this possibility to one side?
I understand this, but two things mitigate against it. Firstly, the hum was not there from the off. It initially played with absolute silence. Something changed. Possibly cables crossing over/being moved? As I say, I’ll explore that once the tree is down. Secondly, there are two reviews which test the amp with high sensitivity speakers and yet zero noise, which was absolutely my first experience.
Bit confused here as the idea of dropping to 1V was initially touted as the solution to possible channel imbalance but now it will also solve the low hum? That seems a bit simplistic?
As per the above, this doesn’t explain why zero noise was achieved and then it changed. Both reviews I’ve seen (Stereophile and Hi-Fi Knights) explicitly address background noise so we can discount the idea I may not have realised it was coming with noise 
Not sure what “… isn’t specifically designed for this kind of speaker” means. The speaker was explicitly designed for valves. Push pull probably better if I wanted to drive to loud volumes but I absolutely don’t given my tinnitus and family listening preferences.
Forgive me a little chortle here. My “prior difficulties” were a mix of a change in my listening preferences allied to the growing evidence that I have zero tolerance for nonsense like “fettling”/rebuilding the system once a year etc. and the evident issues with 4 boxes and 6 cables. The latter were simply never happy and nor was I.
Please explain why the amp is “super tweaky”? Putting aside these initial intrigues it’s less boxes; less cables and fit and forget until new valves are required. Barring faulty valves, total cost of ownership should come in well below regular Naim services too.
Probably also need an explanation as to why an amp designed for high efficiency speakers was “… bound to be problematic”?
Amp self biases…
After all these comments, a quick update…
1 - the offspring is of the view that vocals are centred. This is seeming more like an ear blockage and a Christmas tree. Great theories though 
2 - the speaker cable for the right speaker immediately crosses over the power cable on exit. Without getting too convoluted, I’ve moved it but can’t get it away from the power cable at that point. It’s back where it was when I first powered up but, not unexpectedly, no change to the low hum.
I could move the streamer off the Powerigel, and may yet do so, but suspect it’ll make no difference. Important to do it though because it’s another thing then ruled it.
I suspect what I need to do is move the Powerigel to where the boxed sets currently are (the ones on the floor not the shelf). That will then let me feed the power cable in from outside of the right of the Hutter as we look at it on the photo rather than just around the back and up parallel to the speaker cable.
Anyways, tree down Sunday and then hopefully some time to get methodically stuck in over the next week or so.
I meant the amplifier is not likely designed for such high sensitivity speakers, not the other way round. Did the reviewers use much less sensitive speakers which wouldn’t show up the possible noise? 100dB is really putting a magnifying glass on the circuit noise and will highlight any excess electrical of gain.
Yes, as per comments above. One of the reviews used Klipsch snd Devore.
It was ok in the beginning? It sounds like a tube is going bad. A tube that is bad usually shows up early on.
Noted. Can only be the rectifier presumably as that’s the only one which impacts both channels.
Had planned to slightly relocate whole system today (10cim further out from the back wall and up to 10cm away from the sofa. However, had a bad night so don’t trust myself with delicate physical stuff. Decided to just redo the cabling as best I can in the current position.
Still got the low hum on both speakers when I step up to the cones 
Also still got noise/chatter on left speaker.
Next step is to move from 8 ohm tap to 4 ohm and see if that helps.
After that, a friend has kindly purchased me a NOS rectifier to see if that makes a difference. That one might have to wait until grandchildren day is out of the way tomorrow.
Is it actually anything to worry about if only audible that close? Have the reports of silence that you have read also been based on listening very close to the cones of high sensitivity speakers?
Fair point. Not clear.
That said, it feels like a problem to be solved as there was, as best I recall, silence at first and the left channel chatter, whilst also low level, is similar to the issue I had when my Vonets extender was right next to the streamer. The-TP-Link extender is right behind the left speaker which is chattering.
At this point, the valves have done just over 100 hours and, assuming I can resolve these two small things, it’s very much a keeper.
Friday we tried the 4 ohm tap. Certainly put us in the front row and the PRaT of the very best Naim. It was also far too much and exactly what we wanted ti get away from. Won’t be doing it again. Also, no impact on the speaker hum.
Today we installed the rectifier purchased by a friend. We also returned to the 8 ohm tap. No impact on speaker hum at all but:
- the slight whizzy noise on turning the amp on has gone.
- the periodic burning smell/sense of heat I have been getting consistently, appears to have gone.
- the transformer appears to have developed a hum of its own. Very low volume but there.
- the large transformer at the back of the amp suddenly operates at a much warmer temperature but only on one side.
I presume rectifiers have to burn in as much as power valves? That being the case I’d guess it’s too early to judge comparative sound, soundstage etc?
Guess this leaves me with
- relocating the system stack and giving the cables even more space.
- claiming a new rectifier under guarantee.
- accepting everything as is/querying with dealer.
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You’ve done a lot of testing. Now to get the dealer involved.
Can they bring over another amp to try in your system or do you need to go to the dealer to compare against another amp?
DG…
They’re about 225 miles away and the amp was built to order in Poland. There were no meaningfully closer dealers. Only 3 in the UK. Suspect that unless I pay their petrol no-one is going anywhere and even then I could understand if they were reluctant. I’d be equally reluctant to guess how discussions might play out but we’re past 14 days; we agree we really like the amp as a household and so, worst case scenario, this presumably goes back to the dealer for testing or sending back to Poland and I have to unpack and temporarily reinstall the Naim before we finalise the part ex aspect?
Valves are guaranteed for 6 months so I’ll need to claim under the guarantee at minimum for a replacement. In the meantime, assuming I’m right about the burn in then I’d like to put some hours on this alternative rectifier and see whether I like the sound.
Do you know if the output valves are self-biasing? A change of the rectifier could mean a slightly different voltage is now being applied which would call for adjustment of the bias.
Another question as it’s not quite clear in your early tests - have you tried with every input disconnected? You need to know if this hum is the amp itself and not interaction with a source and it’s earthing arrangement.
Whole thing self biases. 5C3S has been replaced with a 5U4G, which I am told is an equivalent valve. Is it possible for equivalent valves to have different voltages and what’s the likely impact of that? If it self biases presumably there can be no issue with a voltage change?
Have not tried with both inputs disconnected. I only have 2 sources - DAC and TV - and when TV was removed the hum continued just as when the DAC was removed the hum continued.
Have you tried disconnecting everything… Just amp and speakers?
“The-TP-Link extender is right behind the left speaker which is chattering.” these type of extenders are dump all kinds of noise into the power lines
No, have been specifically advised that the amp won’t like that.
The extender is a big step up from the previous one. The fact that only the left speaker is quietly chattering suggests its proximity between the ethernet cable and either the nearby speaker cable or nearby mess of power cables feeding the TV, DVD player and Sky Q.
Latter needs sorting but the priority is the hum from the speaker and now the amp.