Not exactly at -18 db no, but I have done a similar setup yes. You should absolutely listen to KaiS advice. What he is refering to is where Auto reference level will work at optimal in the mentioned setup. He knows all this to his finger tips.
My advice would be to try every way to set this. You learn the ins and outs, and you will find what suits you best. You shouldnt be in a hurry to put this in a fixed setup.
Personally I prefer to go with a locked volume + a suitable reference level for speakers. Set to 3.76v I think for XLR out, example in the manual. For headphones I use Auto reference level, constantly. In practical I dont do much level change with speakers. I often use dim or mute if I need to put my ears elsewhere. With locked volume the unit will become a source like a CD player, and you have a constant output with top SNR. I have PRO so i ran an optical cable to my amp, but Im not impressed by the inbuilt Hegel dac(one of the reasons I got an external dac in the first place)just an experiment.
Not so much discussion here these days, but I would encourage you to make the manual your bedtime reading. This forum thread, you should start at page one and just skim through it all. Use the search function. There are so many brilliant minds here and every question has probably been answered already, twice atleast. ASR is usefull too. Im just a novice like you. I understand math, but don really have the patience or precision for it. So for those who are deep into the numbers here there is probably a 100% correct way to do this, and you should listen to them, but you shouldn be afraid to fool around with it no. Just start at low levels.
I have this setup now, but im looking to buy a poweramp or powered monitors. For that I will use Auto reference level and a fixed attenuator attached to cables to put it in sweetspot for max SNR. I like Cambridge products, I think they are one of the good guys, but I consider a Parasound a23. The new ADI 2/4 is also around the corner, and sadly I dont have a money printing press.
Right now im just hanging around for the ADI-2/4 PRO se manual to release.
ADI-2 DAC, ADI-2 PRO, DigifaceUSB, UCXII, ARC, HEGEL.h80, KEF.ls50, HD650, ie400pro _,.\''/.,_