All wiring and layout typically requires assessment for parasitic characteristics in hi-fi amps. As an example, Williamson laid out his chassis to minimise capacitance coupling from parts to chassis, and to maintain balanced levels of any parasitic effect like capacitance to chassis (especially with respect to the signal paths from PI stage to output stage).
Using any type of screened cable would require care to try and use cable with the lowest practical shunt capacitance, and minimise the cable length, and even then to calculate the likely RC corner frequency and compare it to the phase margin frequency. As an example for the Williamson amp for an 8 ohm speaker output and nominal 3k5 series feedback resistor located at the transformer end, the shielded cable shunt capacitance probably needs to be no more than about 50pF to keep the corner above 1Mhz and hence the start of some phase shift to above 100kHz, although even that could reduce phase margin somewhat if the output transformer was good enough and the forward gain wasn't being throttled in the interest of stability.
In this Golden amp the frequency of the stability margins was not mentioned when using the TO-330. And there would be some doubt about HF resonances with this diy OPT unless measurements were made (which would not be easy), so it will be interesting to see what you can measure Joe. And if you can't see the speaker cones whilst testing then I liked the article's advise to look in the top of the output stage valves and check for 'periodic ionization'
Joe did you include speaker impedance taps, or just wind for one speaker impedance?