I changed out the pop-off valve on my hot water heater, refilled the unit, reconnected the electricity to let the water heat; all hot water dispensing units seem to work fine but not the tub/shower. What's up with this?

When you drained and refilled the water heater, most likely some sediment got dislodged and went straight to your tub/shower faucet. I would guess that removing the faucet cartridge assembly and cleaning it will cure your problem. While you have the cartridge removed, turn the water on for a short period of time(a few seconds) to clear any debree in the line. Then reinstall cartridge, and turn main house water back on and let us know if it solves the problem.