The Bartok actually provides four output level ( line level) settings: 0.2V. 0.6V, 2V and 6V. These are fixed, not constantly variable.
Which you choose depends on a host of things; the average level that your recordings have been mastered at, the input sensitivity of your preamp ( if you use one), the “law” of the volume potentiometer on the preamp , the input sensitivity of the power amp(s) especially if you do not use a pre-amp, speaker sensitivity and how loud or soft you like to listen and so one. So you can see that there is no “right” answer.
In general most people will choose an output level that places the most used range of the pre-amp volume control when the Bartok is set to 0.0dBfs output to be somewhere between from around 9:00 o’clock to 03:00 o’clock. I am not suggesting that you would use all of this range in practice but that the lowest setting of the volume control should be above its physically lowest setting where non-linearities can occur, nor at such a high level that the equipment receiving the output of the pre-amp can be driven into distortion.
If you do not use a preamp and have the Bartok control volume then you do not set the Bartok to 0.0dBfs but to a sensible listening level. However digital volume controls work by discarding bits so I would aim not to have the volume control set much below -20dBfs to maintain resolution.
As well as the above there is simple personal taste. I would start by using the 2V line level output as this was historically the standard voltage level chosen for CD players and most amplifiers etc. will at least have been designed with this level in mind. However, as I said , some prefer other settings.
6V provides the theoretical best S/N figure but with many set-ups this will place the usable part of a pre-amp’s volume control in a difficult to use position or, as you suggest, may overload the input it is connected to. It is probably most useful with no pre-amp and with tube power amplifiers of low input sensitivity. As always YMMV.
Enjoy your new Bartok. I know that however much you enjoyed it at your dealers you will find it even better in your own system.