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There are two main reasons for an amp cutting out - either the power drops to some part of the circuit, or the signal drops. Both can be caused by a component or wiring fault. You've now established that the power supply is ok at the filter caps, though this won't tell you that the various voltages are getting to their destination. You have an extra complication in not knowing that the amp has been built right, so there's an additional step in checking the amp is wired and soldered correctly. There are wires underneath the board, too and these need metering out to make sure they're correct. Unlike a factory-built amp, you can't assume it has ever worked properly.
Make sure that the various grounds are good as well.
There's always the possibility of a bad tube. You've removed two of the power tubes and the problem stays the same. Try try the amp with the other tubes as well; that is, if you tried it with both inside tubes removed, try it with both outside tubes removed to make sure the fault doesn't disappear. The rectifier is OK, but if you have spare preamp/PI tubes try swapping these.
Do you have access to a scope?
Make sure that the various grounds are good as well.
There's always the possibility of a bad tube. You've removed two of the power tubes and the problem stays the same. Try try the amp with the other tubes as well; that is, if you tried it with both inside tubes removed, try it with both outside tubes removed to make sure the fault doesn't disappear. The rectifier is OK, but if you have spare preamp/PI tubes try swapping these.
Do you have access to a scope?
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