Thank you Enzo,

As a manufacturing Plant Engineer, I usually don't have time to do much PCB troubleshooting. I usually just yank the problem one out and put in a new one so the machine is up and running again. Obvious problems like burned or broken components are easily repaired and put on the shelf for spares but intermittent problems are hard to find and often not worth the time/risk to troubleshoot/repair.

However, since this is my own personal amp, I want to repair it.

I will be following your advice and let you know within the next few days exactly what I find.

Again, thanks for your help. It is much appreciated.

Thank you Enzo for the reply. 
Does anyone know of any easy way to check the PCB for a loose/broken solder point.

When turned on amp it made loud humming noise. Checked cables, turned off and on, changed outlets, and still had the humming. I slapped the head and it quit the humming and started working fine. Turned off and on several times over the next few hours with no problems. The following day I turned it on and had the same humming again. I hit the head again and the humming stopped. Have been turning on and off over the past few hours and seems to be fine. What could be causing this? Anyone have any ideas? Thanks.