I own two Oktava MKL 5000 mics and in the info Oktava calls the tube 6C15P. The PSU was factorytrimmed for 4,5V for the heaters. Is there anything to gain by raising the heaters to 5,4 or even 6,3? if the labeling is correct this would indicate that the tube should be used at 6,3V heater, Right? Since I don´t care much for theese mics I really think that almost any change in sound would be an improvement.