But, the passive pre-amp builder, Arek Kallas, made a suggestion that it is better to have maximal (clean) input to the pre-amp and then control the the pre-amp volume down.
So I was sending about -35 db to the pre-amp and it lost something. So I went to 0 db from the Roon source and lowered the pre-amp volume, and what did ya know but it sounds very good.
I suggest this because Roon has the digital PRE pre-amp volume controls – they need to open up for best sound.
Generally I agree with sending the pre-amp the maximum signal, but there is an exception for headroom relating to DSP including upsampling. It is prudent to allow about -2 or -3 dB to avoid limiting from intersample peaks. You can do this by enabling the limiting red signal to the source path and then dialling in small decrements of headroom until the red light stops flashing.
I do all my DSP in HQPlayer and have -9 dB headroom there because I’m also convolving a WAV file. That’s enough to avoid limiting on even the heaviest bass heavy tracks.
I suppose a passive preamp manufacturer would say that, as a passive pre can only attenuate an input signal. The 2V output of a typical digital source is still too much for many preamps, depending on how sensitive your speakers are, and on your preferred listening volume, and it can often be useful to attenuate the output signal in order to get a usable signal that allows more than the first tenth of the scale on your volume control to be used.