Ok - Progress!
Last night I spoke to Richard V. It was very worthwhile, as he explained a number of things to me about the M5-HPs. In particular, he told me that a dead battery will not result in diminished bass output, and that the the problem did indeed seem to be the amp and the input resistance. It was a great call, and took place before I spoke to the amp's builder. Richard felt, as I did, that it was just too coincidental that both the M5-HPs would have their batteries die or fail at the very same time, and at the same time I put the amp back in the system.
Prior to my call with the amp's builder, I snapped a few pics of the inside of the amp and sent them to him. Well, that was the problem: The amp was knocked around a bit by UPS, and stuff had come loose (I had very little idea what I was looking at, but the pics showed everything). Specifically, the transistor bridge was no longer connected to the heat sinks. These of course overheated, and caused a chain-reaction of other issues. So, the amp is going back to have the damage repaired and everything reattached properly.
So, there is light at the end of the tunnel. Diagnosis is 90% of the battle. Now, who do I speak to at UPS to get compensated? ;-)

