My Roland HP237R can only faintly be heard with the volume slide all the way up.
I’d probably start with the volume slider itself, and make sure that’s working correctly. Because the volume slider is both mechanical and relatively exposed to the user, it’s a very likely candidate for failure. I replace them often. Once you’ve evaluated that slider, I’d consider other issues like power supply and amplification, but I’d definitely start there.
Replace all electrolytic caps around the recovery amps just after the DAC - that’ll fix it.