I have a system with dCS apex + Sythesis roma117 dc (tube pre, newly added) +Mastersound 845 mono block (50watt a class SET tube) + Yg Sonja3.2 (recently upgraded from 2.2i) .
The Setting is 0DB from Bartok and when listening
Looks like your preamp has a max input sensitivity of 5V, so the 6V output setting on your dCS dac will clip with some music. The 2V setting is the safer bet and aligns with dCS’s recommendation.
Thank you for the valuable info. I already set the bartok’s ouput voltage to 2V to Roma 117dc.
By the way I have another system;
Rossini apex+ Pass xp32 + Pass x600.8 mono blocks+ yg Vantage3.
I looked up the input sensitivity of xp32, but it only says 42K ohm (xlr).
In this case which voltage output (2V or 6V) of Rossini should I use?
There isn’t one. When dCS offer a choice of settings it is so that the units can be configured to suit the poarticualr system that it is being used in and/or the listener’s personal tastes. This is the same with filter settings.
The idea is that the line voltage setting chosen allows you to set the volume control in a suitable position so that you have a comfortable average sound pressure level whilst still having a range of pre-amp control that can be used. That is it isn’t nearly full on, off or ( for dCS direct coupled to amplifier) falls outside of maximum attenuation suggested by dCS.
Using a pre-amp you must set the dCS volume control to 0.0dBfs. That is full output. Now choose the setting of your dCS unit’s output voltage so that the above condition applies.
The position of the pre-amp volume control is difficult to comment on as different pre-amps will have different volume potentiometer “laws” ( the rate of attenuation or gain). Some have have 1 o’clock as just on, others will not reach this position until well advanced as their just on position is just past 6 o’clock. The position you choose will also depend on the gain of your pre-amp and amplifier, sensitivity of the speakers, size of your room, what genre of music you like and how loud you want to listen.
The main thing that strikes me is that the listening rotation of the volume control you are trying is almost the same but with 3 times the input voltage. I am guessing that you are judging the similarity of volume subjectively. It looks to me like you may have the settings lock selected on your Bartok. Hence you think you have changed the line voltage but have not. Unselect the settings lock via Mosaic or via the menu of the unit’s front panel display first.
Having said the above I suspect that you may find 6V is excessive for a tube pre-amp.
As aforementioned, roma117dc tube pre’ max input sensitivity is 5V, so I changed bartok’s output tp 2v with 0db. Thanks for the comments!
Not sure I understand @Simon2. 2V is 9.54dB lower than 6V, so 2V should give the same loudness as 6V with 9.54dB more gain/less attenuation which generally means clockwise rotation of the volume control. 6V at 1 o’clock sounding similar to 2V at 12 o’clock makes no sense to me unless a) you mis-spoke b) your volume knob works the opposite way to most others (is your pre from Australia?? 
) or c) Pete is right and you actually have settings lock engaged. Confused.
You are right.
I checked again this morning and the numbers were reversed.
For thebsame volume, the settings were
2V-> 12 o’clock
6V-> 11 o’clock !