I’ve just realised, are your subs passive?
You appear to be running speaker cable to the subs first and then from sub to mains speakers…?
Looking closer at @RogerGround system, it isn’t clear which Grado cartridge is in use. From the information on their website the output is either 4.8mV or 5 mV. So, possibly the sensitivity switch needs to be low, or as you say, use the fixed output.
Then to check, is the MC/MM switch correctly set? If, accidentally the MC option is set then there could be an overload of the SN3 input (bearing in mind I do not own Naim equipment).
I’m also not sure about the speakers, are they LS5/9 or LS6? As I read the system list the sub is the Graham passive sub, so the speaker cables from the SN3 plug into the sub which then has an output set of connectors for leads to the main speakers.
Hi Richard.
Yes they are passive.
Hello catswhiskers.
The cartridge is a Grado Reference 1, please attached details.
MC is selected.
The tonearm cable is plugged into the fixed line input on the Quad phono.
The speakers are LS5/9.
Kind REgards.
OK, that does change things somewhat. I note that the sub is pretty insensitive at 84dB/w. And then you also have another set of speakers added on in series, with another crossover, which will surely not only affect overall impedance but also sensitivity seen by the amp. Do you know what sort of load the pairing presents to the amp? I’m afraid I’m out of my comfort zone here so I think it’s probably best to consult with your dealer who doubtless knows this combination much better than I do. I can’t help but feel there’s a mismatch somewhere here in the system.
Those are the specs of an mm cartridge, mc is the wrong setting.
MC setting is not the right one for these cartridge specs. You risk serious overload of the phono stage. You need the MM setting.
Hi Yeti.
I will change the setting and play the same record again and see what happens.
I can only imagine that my old MF Nu Vista amp was such a beast it just shrugged off any overload.
Thank you.
Kind Regards.
Hi Richard.
I will change the setting and play the same record again and see what happens.
I can only imagine that my old MF Nu Vista amp was such a beast it just shrugged off any overload.
Thank you.
Kind Regards.
Hi Roger,
Out of interest have you tried the SN3’s phono input which is designed for ~5mv MM cartridges, like the Grado? The Quad phono amp looks interesting with 6 different settings (3 each for MM and MC).
Hi HappyListener.
No I have not tried the SN3 phonostage.
Foe some reason, and I do not know why, I always thought I had a MC cartridge for the last 9 years. I can only imagine that my old MF Nu Vista amp was such a beast it just shrugged off any overload, with me having the Quad set to MC Medium sensitivity.
Hello Richard.
Would I be able to use a Stageline N with my cartridge?
Thank you.
Kind Regards.
Roger
Yes, a Stageline N should be suitable for the Grado.
Hi Roger,
Out of interest, which MF Nu-Vista model did you have?
It seems some of the MF and other higher current-output amps in the market have more tolerant line-level inputs.
Nu vista M3
The wonders of the internet and having time to search.
There is a review on Hifi-Advice.com of the Graham Sub3 with LS3/5a and Chartwell LS3/5, the design partners for the Sub3, that explains parallel connection of speakers and then that in the design combination impedance does not fall below 7 ohms.
That is the limit of my understanding, although it looks as if it it the MC option could be the culprit.
This is the email conversation I have just had with Graham Audio.
“Would you be so kind as to let me know what the overall impedance is of the LS5/9 and Sub 3 when wired together?”
“Sub 3 is not suitable for the 5/9, sensitivity is to low. Designed to match the 3/5, 3/5A which is 6dB lower sensitivity.”
“I did not now that, the dealer never mentioned it when I purchased them in October 2018.
Having said that I have not noticed any issues, they seem to work ok together.”
"Well, you are the first person to have used this configuration and we are pleased it is successful.
We might look at it as an option.
The rough estimate of the impedance is 8 ohms"
I have set the Quad to MM and just played the record that caused the warning lights last time and this time no issues, dare I say result!
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Thank you to everybody for the their kind thoughts and input and time and trouble.
I have set the Quad to MM and just played the record that caused the warning lights last time and this time no issues, dare I say result!
I am such an idiot, or so the wife tells me!!
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Hi Roger,
Looking at the Stereophile review of the M3, if you used the dedicated phono input on this (pre Quad?), it was MM/MC switchable - but this was recommended as a dealer matter, as the switch was inside the casework.
Anyway, all water under the bridge now.
If you hang around this Forum much, you’ll learn that many parties make errors in connections, set-up, cabling et al - it’s all part of the fun 
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