From your description it seems that the filaments might be powered differently between the two monoblocks.
You might try interchanging the tubes between the two units to see if this effect is associated with the tubes or with the other electronic components.
You should get to the bottom of this, perhaps consulting a technician if necessary.
You might try interchanging the tubes between the two units to see if this effect is associated with the tubes or with the other electronic components.
You should get to the bottom of this, perhaps consulting a technician if necessary.