Sure, my rationale:
a) There are two power cords, you have 2 channels working, so 3 channels failed, but the chance for all 3 amplifiers failed at once is low, that means it is much more probable that it was the one or more of the common components of the power supply had failed.
b) The P5 is a modular mono block design such that each channel has its own P/S transformers and capacitors. So what's the odd for all 3 power supplies have failed? I would guess pretty low.
I don't have the service manual, but I know the most costly parts in such an amp are the transformers, storage capacitors, rectifier bridges, and the output devices (power transistors). Since the probability for all 3 of those things would fail at the same time is low, I therefore would bet the issue was within the lower cost items, that is, the light current electronic devices that controls and/or protect the amp modules. The reason why one of the two channels are working is likely because as I mentioned before, that Anthem was smart enough to divide the 5 into two groups.
I have no idea what ICs TLSGuy was talking about, but I would like to think that the P5, being an older design and is a mono lock based 5 channel high power amp, it won't have much ICs in it at all. Also, you are right, Anthem are still making them so they will have parts even if there are ICs in them that failed.. And any such ICs will be relatively cheap because they is no need to have any complicated in such class AB power amplifier.
If you are considering an alternative, how about sell it to me for $300, you pay for shipping.