I just made some real progress that surprised the heck out of me. I've gone through and done the transistor swaps, added shielding around the inductors and the transformer, added extra caps and grounding. The grounding and capacitors made a big difference. The transistor swaps nearly took care of the ground-modded one, but the other one still hummed a lot. I applied the ground mods and that helped, but didn't fix it.
In desperation, I added a metal box over top of the entire supply section. It helped slightly. I then shielded the other inductor, which made little difference.
Well, the real progress happened when I swapped power supply boards. No change, but for some reason, I decided to move some wires around. Most of the cables made no difference. One made a huge difference---the 6-pin power cable that feeds the right end of the EQ boards. When I moved this DC supply cable as far away from the transformer as I could, the hum basically disappeared.
So not only is almost all the hum I'm seeing caused by induction, a sizable percentage is induced into the ribbon cable that goes from the power supply over to the right end of the EQ boards. This also explains in large part why the 81 hums and the 73 doesn't. Besides having a bigger transformer, that power cable never gets closer than about four inches from the transformer in the 73, but runs almost right under the transformer in the 81.
So it seems to me that the key to reducing the hum may well be getting that power cable as far away from the transformer as possible. Since I don't think I can realistically keep the power cable routed so far away from everything, I'm considering soldering a right angle Molex connector onto one of the jumper cards near the left end and feeding power in from the opposite end of the device. (The power rails run straight across without resistors, diodes, or voltage regulators, and there are decoupling caps between each stage, so it shouldn't really matter which end of the bus provides power.) Thoughts?