Hi, teemuk! Thanks for sharing. It's realy great idea. Please excuse my bad English...I think there is one thing in schematic which can be improved. In that design... "saging" (bias shifting) hapens all the time... i mean from very low levels... practically from zero. But, in real tube amps it occurs when Grid starts to draw curent at high positve levels and charging coupling capacitor, shifting Bias voltage that way. I can't explain it good with my poor language  but i'm tniking for litle corecton in rectifier... something like this will be much more realistic:



The voltage drop at R6, R11 is smal but it can be compensate with more gain in V5 inverting stage.