the PSU starting humming and lights came on on the mother board and the power switch started a slow fade in and out kind of thing.
That sounds just like it were in the hibernate state, but since you took the whole PSU away to be tested, I can't figure there'd be any electricity stored anywhere to maintain that state.
If there were, the solution would be simply to press the power button for about five seconds, until the computer would shut down. Then restart. If the starting problem were due to a locked hibernation, the rig should start normally. I've helped many customers on the phone with that simple trick...
Trying the above doesn't cost you more than five seconds of your precious life, so go for it. At least, the computer should shut down in five seconds and if it doesn't, the culprit usually is the motherboard.
Now, if you can turn it off in five seconds, Captain1ma's suggestion is worth trying. You can try a few tricks while preparing for that, in the following order. Remember to unplug the mains before detaching any component.
- Unplug every USB/Firewire/sound/card reader wires that are connected to the case. Plug in mains and try to boot.
- Unplug the hdd, both data and power, try to boot
- Unplug the optical drive, both data and power, try to boot
- Detach RAM, try to boot. Most computers would beep several times.
- Attach one RAM stick, try to boot. Repeat with other sticks, also with a known good one.
- Leave a known good RAM stick in, detach the video card. Try to boot, beeping expected.
- Try with another video card. Try to boot,
- If the new video card works, test the original RAM one at a time
If the above doesn't give any results, you already have done most of the preparations to take the motherboard and PSU out of the case.
Oh, and yet one thing to check: See if the CPU fan is in the right connector. If it is, try with another fan. Also, if there are any case fans, try with another. I've got an elderly HP mobo here which won't boot without a case fan.