@audioconnection - I started with the preamp volume set to zero, and the protection circuit still engaged, before I had a chance to increase the volume. I am one of those few who RTFM.
I began to think that there was something wrong with the M5-HPs, and they would have to go in for service (lithium batteries installed in 2012, so probably not two dead batteries at the same time). I retained the in-line 10K filters I had used prior to acquiring the M5-HPs for such a situation. I put them in and got more bass out of the 2Wqs than with the M5-HPs set to 10K. That is puzzling.
I raised the bass level on my preamp (those rarely used tone controls do come in handy!), and got sufficient, if not ideal in quality, bass output. I was able to determine that the 2Wqs are indeed working and generating sound. I got similar results with multiple source components.
What this means is that, I think, the new amp is not putting out much bass. Hopefully this is a burn-in issue, but I will confirm with the amp’s builder today. Of course, this does not explain why the in-line filters offered so much more bass output than the M5-HPs set to 10k.
If I am wrong about the amp, I will give you a call, Johnny. Thanks for all the assistance. Ditto everyone else here.