Many thanks to all who responded. As I hoped, I learned a lot.
First, I can count, notwithstanding that I said I would ask three questions and posed four. That’s what happens when you think of one last question just before you click the “submit” button, without bothering to re-read what you have already written.
Here is an update and some additional information about the power cord in question.
Update. The lights in my kitchen and dining area are on the circuit that I previously mentioned has been acting up for the last year. When I went to bed last night, those lights were functioning, but this morning they would not turn on at their switches. I have been “fixing” this problem by flicking the breaker on their circuit, but this morning that did not work. Before I started this thread, I did some research about circuit breakers in the archives and learned that repeated flipping of a breaker will wear it out. I guess that is what has happened to the breaker controlling the lights in question. I don’t blame myself for the current (no pun intended) situation, because the only alternative to flipping the breaker is to permanently stumble around in the dark. I’m going to have the landlady send around her electrician to replace the breaker, but while the electrician is here I’m going to ask him (no offense intended, Elizabeth, but the electrician is probably going to be male) to look at the breaker on the circuit to which the amps were connected. The age of the panel (over 30 years) and the failure of one breaker suggests that the breaker on the amp circuit might be suspect. I’m still puzzled why only that breaker tripped when the power went out and came back on.
Outlet for amps. Gs5556 made an interesting point about not putting to much electrical “stress” on a 15 amp circuit. In the future, I will connect each monoblock to a separate outlet (circuit). My choice of outlets for the amps shows how, in the absence of specific knowledge, you can be trapped by your own thought process. When I had the outlets rewired to create dedicated circuits, I planned to have one 20 amp outlet (for the amps) and one 15 amp circuit (for everything else). Porter Ports are rated for 20 amp, but that requires a 12 AWG wire, which could not be installed without tearing out the wall (not an option in a rented apartment). The electrician had to work the new wire through the wall cavity using the same sleeve as the existing wire and a 12 AWG cord would not fit so another 14AWG wire was used. Notwithstanding this change in the amperage rating of the outlet, and not knowing better, I stuck to my original plug arrangement.
Power cord. Despite the request of several posters, I am not going to disclose the maker of the power cord because I now believe that my deployment of the cord may have had something to do with its failure. I will say that this maker sells directly, makes cables to order and has an excellent reputation both as to the quality of his product and the way he does business. I have not discussed this matter with him, but will do so shortly. The amp cords had been in continuous use for 2.5 years before this incident with no problems.
My amp power cords are only about 3 feet long, because my equipment rack is up against the wall and, even though the rack is unusually deep, the distance between the amps and the outlet is short. On a stiffness scale of 1-10, I would say the cords are a 7.5. They flex easily but they retain the shape into which they are put and will stay elevated in mid-air rather than drape over the rack. It turned out that even 3 feet of chord was too long for the distance between outlet and amp, so the cords had to be shaped in an S curve. The manufacturer did say that I had to be careful about bending the cord to avoid damage. I took my time and made the curves as rounded as possible, but that is a lot of curvature over a short distance. The shape of the cords remained as originally set up, so they were not subject to repeated flexing. The damaged cord is now coiled, so I can no longer tell whether the place where it failed was where it was flexed. The cord jacket is mesh and the conductor is silver. I don’t know if the conductor is stranded or solid, and I have no idea what the insulation is.
One option I am considering is to send the cords back for shortening so they can run pretty much straight from outlet to amp. To obtain maximum straightness for each cord, the length of the cords might be a few inches different, which I don’t think will make a difference in their performance. I am sure there are some who will advise not to use either cord and start all over. I'll discuss this with the manufacturer as well.
In response to some other specific questions and comments
1. No pets have been in the apartment. Also, no small children.
2. Jea48. The dedicated breaker DID trip off on this occasion. The other breaker, which controls the kitchen and dining area lights, had been going off for some time.
Again, thanks to all who responded.