I have used the RME ADI-2 FS as DAC in my system since May 2019, with this gear and signal path:
AudioPC >> ADI-2 DAC >> Naim 202 pre-amplifier >> Naim HI-CAP PSU >> 2 x Naim NAP250 power-amplifier >> Audiovector R3 speakers. So 2 x NAP250s used to bi-wire the speakers.
With Naim's special way of connecting components the signal is routed through the HI-CAP PSU which provides power for the 202. With this set-up, Auto Ref Level ON, ADI-2 set to -10dBr volume and +7dBu reference level, the 202 volume control operates around 20% from start. The (vintage) Naim NAP250s, from what I can find out, has specification of 1.6V and 24dB gain.
While looking for tips regarding filters and PEQ I came across posts on using ADI-2 FS as pre-amplifier, and this discussion on level matching. Given the quirkiness of a Naim system I was wondering if the ADI-2 could drive the vintage NAP250s and the bi-amping configuration, and only way to find out was to give it a try!
Playing the first track was unsettling as there was a significant change in the overall sound, with first impression of 'something' missing. But then, after playing different tracks and genres came the realization that a significant amount of coloration was no longer there. With the Naim set-up, when listening at 'home concert level' (when wife is out) there is a certain compression-like feel to the sound which gets uncomfortable after a while. With the ADI-2 as preamplifier going directly to the NAP250s the sound is scaling to amazingly high volume with wider soundstage, higher resolution and better clarity and importantly, very musical overall performance. It is not often during 40+ years in hi-fi that I have had such a profound eureka moment!
So using the ADI-2 as preamp is the way to go, which brings up the balancing theme. Listening at normal level (music, TV), with Auto Ref Level ON, the ADI-2 volume setting is around -40dBr. When I listen to music at the home concert level the ADI-2 volume is in the range of -25dBr to -30dBr with reference level of -5dBu. The sound stage is rock solid and no distortion audible.
So the question is if this is ADI-2 operating within its high performance envelope, or if using attenuators will provide further optimization. The NAP 250 has 1x XLR connector which is connected to RCA out on the ADI-2. So RCA attenuators like the JTS MA-123 can only be connected to the RCA out on the ADI-2, while I understand from your initial post MC, that the MA-123 should be connected to the NAP250 input, which is not possible with current interconnect cables.
Input, advice and recommendations much appreciated!