Does your ARC 150 amp have the upgraded and lowered main circuit board mod for KT150s, and with more clearance for the top cover?
I remember reading some (not all) of the ARC tube amps around 2015 having issues with KT150s and main board changes, some failing resistors or something, among other things got redesigned. Bias issue and close matching tubes might not be as much of an issue after mod, I don't know. Also, comments about improved sonic quality and longevity.
Seeing redesigned amps by different brands with KT150s past two years, and suspecting maybe they figured out how to drive the tubes differently, fwiw. I wonder if ARC might offer any tweaks for them today?