What are the improvements that can be made in the excellent AVM Evolution PA 5.2
Sorry I missed this guys. The stock tubes from AVM are good sounding and extremely quiet but are not great in terms of detail and imaging.
Staging is enhanced with good NOS tubes in every direction. I find depth is most enhanced by Mazdas. Mazdas are also quite detailed and tonally relatively neutral and flat from a frequency response perspective.
Mullards are slightly rolled off in the treble. Big staging in every direction and strong levels of detail with a richer midrange.
I have not tried other options. I have since sold my PA5.2 and those are the only tubes I rolled in. You could reach out to Tube Depot or Brent Jesse to see what they have to say about their options for ECC83s.