Dear Martin,
Thank you so much for your thorough explanation! I am absolutely positive that I have XIb (even though the plate on the back of the cabinet says otherwise), since I have the original purchase documents and also the receipt from the NATO repair shop that repaired them once in 1976.
As far as for the sound, I think they already sounds fabulous as they are, I managed to restore one of the front grille (left) and enjoying them a lot, I power them with a SAMSON Servo 120a, the signal feed comes from an analog mixer (in the picture YAMAHA MX12/4 to be replaced with Alle