Help Diagnosing Headphone Output Issue – Marantz HD-DAC1 (CUP12689 Board Suspected)

[Revoxis]

Hi all,
I’m working on a Marantz HD-DAC1 that has two persistent issues with its headphone output:

1. Colder, flatter sound with noticeably less volume and body (3 to 4 decibels less) compared to a second, identical unit I own.
2. Audible static/hiss at 100% volume with no signal playing — not present on my other unit.

I connected both Marantz put both volumes knobs all the way to 0, then used one of the remotes to increase the volume to 25% (Both volume controls went up together) which clearly let me hear the difference in volume.

What I’ve done so far, taking boards from my good unit:

• Swapped the DAC board (CUP12688Z)
• Swapped the power supply board (CUP12689-2)
• Swapped the headphone amp/bias board (CUP12689-3)
• Swapped the front panel and logic boards (CUP12687-2 and COP126878)
• Swapped the transformer (CLT5P055ZE)

None of these fixed the issue. The only original part remaining is the mainboard (CUP12689).

Headphone used is a Beyerdynamic DT1770.

I believe only the headphone output is affected, when connected through RCA output they sound identical.

• I did replace C958 and C968, but didn't make a difference.
• Bias is set correctly and matched between channels

Any help tracing the signal or suggesting next test points would be greatly appreciated!
Photos available on request.

This is the service manual i'm using -> Marantz HD-DAC1 Service manual

Thanks!