[Argus25]

At least in this case it is not a bad idea to use the design of a precision lab instrument, like an oscilloscope vertical amplifier, as an audio amplifier. I have seen much less impressive ideas from the audio field.

The design of this sort of amplifier of course was intended for good waveform reproduction on a CRT's screen, especially for square and other wave forms, where you want to see the original wave, free from amplitude or phase distortions. So its not a bad ethos for audio too.

The effort that went into these scope amplifiers focused on the knowledge that if the group phase delay for all of the signals in the bandwidth of interest was the same, and the amplitude of all individual frequencies that were being amplified was the same, then there will be no (or very minimal) harmonic distortion. So "distortion free" or high fidelity amplification is just another way of saying that an amplifier is free from errors in amplitude and phase.

To make this clear for the OP I have attached a page about it.

The only thing in this application that to some extent sabotages the notion of doing it for audio, as Radio Wrangler pointed out; "The output transformer had to go", is the phase shift due to the transformer's low frequency response limitations and resonances in the output transformer. That can, as noted, cause instability when the negative feedback is applied.

One thing that might help if there is low frequency oscillation with the feedback applied is to make sure the time constant of the few grid coupling caps that are there is large and go to higher value capacitors to help maintain the LF response because the cumulative delays due to RC coupling add up and the negative feedback at low frequencies can go positive and your amp could oscillate.