Hi Nick, I cannot salute your match-up enough. Penta 3 love the BC 9300 and I like the sound of your streamer connection with Apple Express too!
Regarding the stand-by opton it's something B&O designed into their products from this era, so it should be the best way. But I too go for periods where I shut down everything for a few days (Penta 2 and BS6500), often midweek, to allow components to cool off. Whether it's right or not I don't know, but it feels like I'm giving the boards and caps a break.
I've no idea whether it's effective or not though.

Cue the next responder....

Great set up, as pepps said!  I also use Airport Express, although currently plugged into a BeoSound 1 (older CD/radio type).

Although I currently have the BC and Speakers on the same plug I will change that so I can leave the BC on standby to maintain the saved settings, although it does needs the battery changing now.  I will put the Pentas on a remote switch so I can rest the power supply when not being used.

 

I am working through re-capping and foaming my other Penta set and then in turn this set, to refresh them for another 30 years.  So I want to give them as easy life as possible hence turning them off.  Encouragingly, after 30 years of use the pair I have re-capped showed no signs of significant degradation, the max was 20% variation on the nominal capacitance of some of the capacitors, but no bulging, weeping or heating.  What I have seen though is the mid range L3 coil on the crossover degrading by possibly getting hot, so I have ordered some new ones and will change them too.