The basis of this method is looking at feedthrough from the driver and reducing it to a minimum value. I used the method with power triodes such as 250 th's all the ime. ou are adjusting for MAaXIMUM negative feedback. To say the tubes must be powered shows you don't understand the concept, but that you have all the answers.
So why must neutralization be performed whenever the output tubes are changed?
As it states in the 902dm Service Manual.
TC01 is the neut cap, providing that negative feedback from Plate to Input of the power tubes.
The interelectrode capacitance can differ from tube to tube set, among a few other parameters, and I fail to see how that can be accounted for if the tubes are not present.
Then there are the various Sweep Tube arrangements, unlike the venerable 6146, I've seem some of those darn things able to takeoff all on their own.
Whatever, the Service Manual clearly explains first tuning up at 29MHz, finding the right side of the dip, and then adjusting Neut Cap for lowest Current figure *while the thing is transmitting into a dummy load* - and I have found that to work just fine in quite a few different hybrid rig situations over the years.
But really, to state that I don't know what negative feedback is, well, either puerile or puzzling, pick one.