RME ADI-2 direct to amp

OP I don't own this dac yet but I'm researching it.

Sometimes the Dac designers when they offer Dsp (volume control and such) on the Dac  will account for not losing resolution and the RME is a well designed dac. I think you would like to stay at -6 to -3, to avoid possible clipping and not too low -10 and more allowing the extra headroom to increase possible noise floor. Maybe read the manual since I heard it is very complete.
Did you purchased the pro model? You are biamping and I also read the pro model with dual chips have independent channels for 2 possible amps, one balanced one single ended, I am biamping (planning triamping too) and thought you might be interested.

In regards to the preamp question I have tried dac direct with a project S2 and Denafrips Terminator in my system using hqplayer Dsp (with Denafrips) as volume control and I preferred a preamp (preamps) over it, the choice of going direct I think it is very dac dependent (on its output), it seems the RME benefits this design but I am not completely sure since I haven't tried it.Some others have used analog stepped attenuators controls (gold point) with dacs to avoid the preamps and digital Dsp and preferred this too but it is a gray area some experts say the attenuators have their own tradeoffs and well designed digital Dsp (especially when you are already upsampling like me with hqplayer) are better. I choose to believe these are, if it is real or imagined I have no idea, it sounds good enough to me and the math behind it makes sense.