SMSL AO200 / HO200 noise issue at volume above 47

I got a new SMSL stack (ao200, ho200 and do200).
I am experiencing some noise issues.
The DAC is connected to the headphone amplifier and I use the headphone amplifier as pre-out to my speaker amplifier.
Everything works really well (up to vol 47 on the speaker amp).
The issue is, that when i turn the volume on the speaker amplifier (AO200) to 48 a lot of noise occurs.
At volume 47 no noise at all.
When i connect the speaker amplifier directly to the DAC (DO200) i can adjust the volume up to max. with no noise at all.
But as soon as i have the preamp (HO200) between, then the speaker amplifier generates a lot of noise when changing the volume from 47 to 48.

When testing noise connected directly to the dac, the dac was set to max output volume.
When testing noise connected to the HO200(pre-amp), volume change on the HO200 had no influence.

To me, it seems like an pre-amp output issue but i dont understand why it occurs when changing volume on the speaker amp(AO200) to above 47.
No noise at all from headphones btw.

EDIT No noise when units are cold.

Anybody who can explain this to me?

Best regards, Kim