[Stereocraig]

Bypassing the amp and power inserter, is indeed a sure fire way to determine the condition of the amp.
Although I would expect it to be blocking all signals if it's dead, I suppose it's possible to have been only partially damaged.

Another little known trouble spot, is the grounding block.
If there is even the slightest moisture, or oxidation inside it, or the F connectors, it will be susceptible to arcing.
Carbon arcing can't be cleaned, so anything that has them, will need to be replaced.
Spider and cobwebs, will attract overvoltages, too.