KT120 for Primaluna or Mystere?

Would anyone know why Primaluna and Mystere do not recommend KT120 tubes as Kt88 replacements? Is it because of their auto bias feature?
I would assume it has something to do with the circuitry rather than the auto-bias feature. I would contact the PrimaLuna rep for the US, I believe his name is Kevin Deal. Don't have the contact info in front of me, and I have never needed to contact them before, but I hear he is great to work with, and very good at answering such questions.
yes Kevin Deal at Upscale Audio