I found a passage in the Audio Note Kits manual that states that 'for 15 seconds after power on the amplifier will hum as DC current stabilizes but it should disappear completely'. With ordinary WE style 300Bs this translates into a low level hum from the amplifier which is transmitted into the loudspeakers. I'm guessing the 300B XLS is more powerful and apparently more sensitive to this transient DC ripple, thus the loud howl transmitted through the loudspeakers. KR website states that this tube has 4 filaments and 32 'functional cathodes'. Perhaps the construction of this tube makes it ultra sensitive for those brief 15 seconds. (and it does stop completely)
Also the Texas Instruments LM-1084 5V DC voltage regulators in these amplifiers are rated at up to 5A, with 300BXLS filament draw of 1.9A. it is a parallel single ended monoblock but there is a separate voltage regulator for each tube. I guess it sounds safe enough to go with the better sounding KR 300BXLS as long as the 15 second loud howling/DC settling noise doesn't damage my loudspeakers.
Also the Texas Instruments LM-1084 5V DC voltage regulators in these amplifiers are rated at up to 5A, with 300BXLS filament draw of 1.9A. it is a parallel single ended monoblock but there is a separate voltage regulator for each tube. I guess it sounds safe enough to go with the better sounding KR 300BXLS as long as the 15 second loud howling/DC settling noise doesn't damage my loudspeakers.