Adding a new tube, a 6BK4 in place of a 6BK4 B created more brightness in my ctc 17x ad, and some bluish glows in old tubes and some popping/clicking sounds (it hasn't made any sounds up to this point). It is a HV regulator, so is the set getting more volts? As in perhaps closer to the 24kv it needs? And older tubes glow more?

Are these interchangable?

Not interchangable, need 6EL4 or similar