I'm back again, with a metamorphosed amp. Yes some of the cables are longer than they should be however, they are mostly and have all been braided (the ones that aren't braided are not for testing reasons.)
Well I have some good news and some bad news, however I hope someone may have answers for some of it.
As you can see it is entirely point to point over a ground plane (zinc and foil). All cables have been kept as short as possible.
My main issue at the moment is a high pitched noise (around 8kHz in the left channel). This is the side with the two coupling capacitors, I don't know if they could be considered the issue, but no signal flows directly next to them. I still have a fair amount of other background noise (even when in grounded metal box) which may (probably) be due to the smps as it's about the same in both ears. I could live with it if I managed to clear up the uneven and very wearing noise in the left channel.
Do you think that the capacitors are likely to be a the left channel problem? And if yes, how should I go about resolving the issue?
Thanks for all the help I've had so far,
What do you mean when you say the left channel is the "one with the two coupling capacitors" ? As far as I can see the circuit is completely simmetrical.