What input on the STR do you connect the Lumin to? I imagine it’s what ddude003 mentioned, according to Lumin.
If you bypass the DAC in the STR you bypass ARC room correction.
Some systems (perhaps with a high-gain pre-amplifier) may be better matched using the ’Low’ setting. It uses a combination of DAC settings to lower the output level by around -4db.You’re just giving it some headroom it’s probably clipping on the transients.
If you bypass the DAC in the STR you bypass ARC room correction.