WoW is all I will say about that build...
The power amp heatsinks should have the fans...otherwise it's pointless...
The wiring is lying in stray fields all over the place, and it's not shielded where it needs to be...
You go to all the trouble to glue up your cable ties etc...but park everything else in there like a spagetti vomit on a saturday night...
Personally I'm not surprised you are having trouble with it.
Does it not mention anything in the manual you read about construction and wiring etc ? If it does, I can't believe they wanted it to be done this way...
I admire your enthusiasm with regard to building your own amp, but in order to be really proud of what you have achieved, you need to pay the same attention to every detail of the build from the metalwork, to the wiring and everything in between.
Sort out your entire build, then diagnose any faults you may have with it's construction or design.
Whenever I build equipment, no matter what it is, I start off by thinking if this bit goes wrong, or that bit goes wrong, how easily can it be serviced. Once you have chosen your easiest route to servicing everything in the box, that will determine where each bit should go, only then can you make the odd compromise based on other factors, like the preamp and the power supply are right next to each other, or this bit or that bit needs screened etc.
as an example of what I might have done with this particular build...
put the power supply right in the middle, at the back of the case with large meshed vent holes in front of it. and put a u shaped shield round it with a hole in the middle at the front for the wiring to pass through...arrange that the shield does not go all the way to the back, but rather leaves an air gap.
The power amp modules I would have put on either side of it with the heatsinks towards the back of the case, so that they are at least level with the power supply shielding, the fans would go in each corner at the back, and be blowing outwards, thereby drawing cool fresh air in through the vents across the heatsinks and back out through the fans...everything is kept nice and cool then. Another shield the full width and height of the case gets put right across the front of it all with the pre amp in front of that...etc etc
this gives you somewhere handy to hang all your wiring from...
All of the signal wiring would be shielded, all of the heavy power supply wiring would be routed as far away from the preamp as possible, both would take a different physical route to their destination and anywhere they cross each other should be at right angles. Speaker outputs would be at the rear, the returns for which would be as short as possible from the common ground point at the power supply as would all other ground points etc etc
I could go on and on, but you get the idea...
Now go away and have a rethink on your design...
I know I'm being very critical of your build, it's true, but don't take it personally, I'm actually trying to help you understand that there are better ways to go about this sort of thing...