How loud can I listen to my P3esr without damage?

Without damage to what? The speakers? Or your ears??

As long as you don't drive the amp into clipping there should be no problem.

Without damage to what? The speakers? Or your ears??

to my harbeth.

They should be fine.  Out of curiosity, I got out my SPL meter and played mine at that level at about the same distance, using my Vincent SV 237 integrated, which is 150 WPC.  That's a little louder than I listen, but is more likely to damage your ears than the speakers.

I would be more concerned about long term permanent hearing loss 😎