Audio Research VS115
yes, turn down the bias to about 50-60% of what it was driving the kt120’s, and then insert 6550/kt88, then turn on, wait 5-10 min, and bias up from there to the target 65 mv across the test resistor
https://forum.audiogon.com/discussions/audio-research-vs-115-bias-adjustments
a good reference post for you, op ^^^^