That sounds an awful lot like a short, causing the power supply to shut down.
But it wasn’t attached to the case. just sitting on a wood table with only the psu and the speaker attached.
I think you need to enlist the help of someone with a voltmeter and some PC-builidng experience…
Sigh. My dad has one, I’ll ask him about it. And I used to think I wasn’t a complete idiot with computers 
I feel pretty confident in working with them but this has just completely made me feel completely stupid.
Well, it sounds like you did all the right trouble-shooting steps. Sometimes it just takes another pair of eyes. Don’t be annoyed when the answer turns out to be something really, really obvious - it always works out that way!