I bought a new power supply (XP Power, “medical grade”), with the same specs, and it seems to have solved the problem. It also reduced low-frequency noise somewhat. Here is a sonogram from the same channel and settings powered with the new PSU (left) and the original one (right).
SSL made a great decision to put an external, and fairly easy to find, PSU ! But they were probably a bit unlucky with the supplier. With the current covid restrictions, doing quality control in China must be close to impossible. I would have paid maybe 1k more for it to be made in the UK. Quality control is better when the engineers can visit the production line. Nothing wrong with Chinese brands making products in China though!
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