Many people want to try the 6V setting but are worried about overloading their preamp’s front end.
At least for the Ayre KX-R Twenty, we have an official statement:
I asked about 2V or 6V:
The KX-R will work just fine with either of those output levels. Let me know if you have any other questions and I’ll be happy to help.
Best Regards,
Ryan Berry
Ayre Acoustics
Yes, I tried that in my system with no problem.
I had a similar reponse from Music Fidelity with regard to the NuVista 800:-
‘It will be totally safe to feed 6Volt into the XLR inputs of the NuVista 800. The limiting factor here is the input selector chip, that will start clipping around 18V. The rest of the circuit board (after that chip) could even take more.’
I asked ATC about compatibility their SCA2 preamp and dCS output gain. They replied that in order to get a better Signal/Noise, volume control knob should be set between 10 and 13 hours. In my case it’s 9-11 hours at 2v.
With 6v, the volume level of many records is too high for this range in my room (18m). But it sounds a little better than 2v))
Ditto when I use my SCA2. The setting of the volume control on my EAR Yoshino 868 at 2V DAC output is even lower for a comfortable sound level. However I do realise that my listening level may be lower than many as I listen mainly to chamber music or smaller orchestral works. That is a mean SPL of around 55-60 dB
As the designer of the SCA2 and the ATC tri-amps on the 50As pointed out to me on a home visit " You don’t even get these out of class A ". If I did I am sure that I would get a response from the neighbours as I guess in this case that could mean SPLs of over 100dB .
55 years of experience in this hobby has taught me at least one truth: It is always a compromise somewhere.
That can be a concern, especially with preamps with conventional volume controls.
Preamps with more specialized volume controls are not prone to this.
What you may want to play with is whether you hear a sound difference with the volume control on the pre turned down and the dCS in 6V mode, and to compare with the preamp volume up and the dCS volume control turned down a little.
It may be weird intellectually, but as always it costs nothing to try it and see whether it makes an audible improvement in your system.
This is actually harder to do correctly than just using the volume controls on DAC and preamp and judging the SPL subjectively.
There was quite a lot of research I believe in the 1970s, which established that very small volume differences misled listeners into always preferring the louder one even though all other factors were identical. These differences in volume could be a small as a fraction of a decibel ( e.g. 0.25dB which is stated by professional recording engineers as the lower limit of a detectable level change by them when comparing mixes. I suspect that average listeners would not necessarily be directly aware of such a small change even though it may register subconsciously).
Hence you should 
find when reading articles comparing , say, amplifiers in reputable magazines that the testers state that they have equalised the volumes to within 0.xdB.
To do this an acceptable music listening level is chosen on one component. Then , without changing the level, a test tone is substituted for the music. This is necessary as the next stage involves measurements. The voltage produced at the output terminals is measured and, using the same tone, the volume control of the second component in the comparison is adjusted until the same voltage ( i.e. +/- 0.xdB) is measured at its output. The volume control position is then marked on the component ( masking tape is handy for this). Of course if wished this could be repeated at various SPLs to give a range of comparable settings.
Without using such a regime I would be hesitant of judging that a 6V voltage output is necessarily better than a 2V one when producing the same SPL through the system as I doubt that I would be able to set a comparison within 0.25dB or better reliably by ear.
Only if you want to take a rigorous science-heavy approach.
Personally, I don’t do double-blind or ABX tests, I do sighted listening with volume levels around the same.
Unscientific? Perhaps, but the things I’m listening for usually aren’t impacted by volume.