My problem has been solved. I assumed that sending a CC22 would change the Powercab's preset which would indicate the selected speaker in the display. What was happening is that the display would indicate <edited>. I'd had trouble getting Logic and Powercab Edit to open at the same time and thought that they weren't happy sharing the USB/Audio connection. Thanks to some advice from Tom Baeppler, a Guitar Product Specialist for Yamaha and Line 6 here in Melbourne, Australia, I persisted and was able to see in Powercab Edit that, although the Preset name didn't change from Jarvis, the speaker did change to an Essex, as did the microphone. (Powercab Edit would then freeze and lose connection with Powercab straight after).
I realized afterwards that I could have seen the same speaker change and mike info on the Powercab's display had I looked. I could also have used my ears but was so busy trying to solve the issue I hadn't actually been playing the guitar - I should have taken Frank Zappa's advice - Shut Up And Play Yer Guitar!