This was the topic I picked that up from: https://forum.rme-audio.de/viewtopic.php?id=27767

This was the particular quote:

"Using -5 dBu with volume down to -45 to -35 means that your Quad power amplifier has an extremely sensitive input - or you listen at very low volume. If you like it and don't hear noise or hum you can just let it be like this."

I assumed from that... that in a similar scenario there are times when you could hear 'noise or hum'

I think you're missing the point by being preoccupied with noise.  That's not really the question at the heart of this.

What I really want to know is which settings would be good for my amp. 

On a different thread a different RME owner with a different amp was told if he is getting noise at low volume levels they might go away with different settings.  That's not to say my only concern is noise... I get a bit of hum from the system while there's nothing running through it... if that happened to go as a consequence of programming other settings then all the better... but not the primary focus of my question.

Like I said, it's not the most important part of my opening post.  If that wasn't clear then I apologise.  But it seemed pretty obvious, the main purpose of asking the question is even in the post subject.

If it helps, let's imagine I do have balanced cables... then we can move on to the next bit?

No.  I just checked, without the RME connected to the AMP the system still hums a little, not the worst I've heard by a long way.  Reconnecting the RME doesn't change anything.  So nothing to do with the cable between the RME and the AMP.  I'll check for separate solutions to that problem.

Now, onto good RME settings.  Can you help?  Or comments from others also welcome.

If balanced cables are not supported in your case and if you have hum, then you can try to reduce the hum by i.e. making sure that ADI-2 DAC and the NAIM are connected to the same power strip, so that ground level doesn't differ too much.

The things mentioned in the thread that you found is about using the proper reference level between ADI-2 DAC and your power amp to
- have no distortion (if the signal should be too hot for your amps inputs) and
- to keep SNR low

Ideal would be on the ADI-2 Pro to use volume settings close to 0dB, but this will be far too loud.

To make this better possible you can do the following:
- choose a lower (fix) reference level on the ADI-2 DAC, cinch outputs have already a lower output level
   compared to balanced outputs, see manual
- use switchable -20 / -30 dB attenuators
- the rest you can perform with the volume knob to reduce volume maybe to -10dB

I think alternatively you could also use auto-ref-level on the ADI-2 Pro and you need to look up how far you can turn up the volume level (the display tells you the current reflevel) but this could quickly become too loud, you should definitively get attenuators. You do not need to take the most expensive ones, I got some from Thomann which can be switched between -20 and -30 dB. Based on information in this thread: https://forum.rme-audio.de/viewtopic.php?id=25399

Emmett,

The User Group has given you some very Solid advice regarding your matter.  Follow it!

Your RME has a noise floor that is astonishingly far beneath the threshold of audibility.   Hums,Buzzes, and other such problems are almost invariably caused by problems elsewhere in the Signal Path.

In addition to the good guidance you've already received, look into your Cable "Dress"    This translates into keeping Unbalanced Signal Cables (like yours) AWAY FROM Mains Cords, Transformers, etc.   If they must cross paths...do so only at Right Angles to minimize the chances of induced noise voltage.   Messy, Rat's Nest, tangles of HiFi wiring causes problems!   So do Dimmer Switches...   An Audio Nightmare.

Good Luck!

Curt

Got the attenuators and plugged them in. Set to maximum.  Sounds much better.  There's actually a full range of sounds coming from the speakers now.

I am that other user with an amplifier that had an input that was too sensitive (a Quad 606-2 with 0.5 V sensitivity). I was advised by RME to use inline attenuators and those did indeed do the job. Auto reference level setting is on. And the sound is excellent. No hum either.