Building a 300B PSE

[John Caswell]

I am a bit puzzled by three things
1) I would have expected that when you swapped the leads to the speaker terminals it made a difference. I would have expected it to break into violent oscillation - so that makes me think that the feedback is correct.
2) This "singing" - as you have a dummy load on the amplifier where are you hearing it from? Is it coming from the output transformer? Does it occur with no signal? If it is in sympathy with the input signal ie it sounds like the 1khz test tone, then the transformer is ringing with a phenomenon known as "magnetostriction" whereby the windings and transformers are changing size, albeit very, very small at the same frequency as the test tone.
3) I suspect that you have the humbucking pot wired incorrectly.

John