Sounds like bad solder joint(s) to me.. Easiest fix would be to send it back to the original owner and ask him to fix that Or you could try to reheat the solder joints and see if that fix the intermittent LED problem. Swapping both tubes is also a good idea, since most problems can be traced to a bad tube.
BTW you might have known this, but be careful when tapping on things while the amp is on! There are high voltages there. If I have the correct picture in my mind, you are pointing to the first cap in the power supply just next to the diode bridge, and if I remember correctly that cap has about ~200V on it.
I do not have any schematics or assembly manuals, not sure if these are available online.
Any advise woudl be appreciated.
Again, the previous owner is your best bet here. As far as I know, the manual is only distributed in a CD when you buy the kit and is not available online for copyright reasons.