HD3S hum problem.

[Rhod]

Hello, I'm new to the forum and to DIY audio so please forgive any ignorance or lack of etiquette as regards to both!
I built my HD3S xl and it worked beautifully for about 5 hours then developed a low hum through both channels, independent of the volume control. I've put shorted phono plugs on the inputs, tried it disconnected from everything except power, checked any earthing connections I can find with a multimeter, done a visual check for any loose wires, solder bridges and generally poked around with a chopstick to see if anything makes it better or worse, all to no avail. Could it be a component in the power supply or still an earthing issue? This is my first proper build (previously just a few guitar effects pedals) so I'm pretty much in the dark and any help would be much appreciated as the amp sounded great before this started.