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6 dB difference is completely normal. This is becauase DSD 0 dB level is defined to be 50% of the theoretical maximum. And the specification allow temporary peaks up to max +3.15 dB DSD level. While PCM has hard ceiling at 0 dBFS.
Some DAC chips like ESS Sabre attempt to normalize the output volume by applying 6 dB gain on DSD. Then PCM and DSD have same 0 dB reference level. But that has a risk when some material actually exceeds 0 dB DSD level and goes to +3 dB or so. Then it may hit maximum voltage span of the analog section (for example if it's running from 5V supply)...
TI chips have varying DSD output level depending on selected DSD analog filter. But typically have about -3 dB level compared to PCM 0 dBFS level. AKM chips Direct DSD output level is -3.5 dB compared to 0 dBFS PCM so they have enough headroom to accommodate the allowed DSD peaks.
Because of uncertainty of behavior above 0 dB DSD level, for example HQPlayer is trying to enforce output to stay within 0 dB and don't allow output to exceed the +3.15 dB level.
Glimrende. Takk for tips!