Backfiring on approach

I think you have the answer -  Reduce throttle slowly (don't just go from cruise power, to idle in one swift movement) This is good practice whatever engine/aircraft you fly.

I'm not making rapid/abrupt power changes.  This is the fourth 912 powered aircraft I've owned and is the only one that has exhibited this behavior . It only does it when you come from a low power setting to idle.     

Fair enough.

To the best of my amateur knowledge, backfiring/preignition can only be caused by:

Incorrect ignition setting.

Hole/crack in the exhaust system.

Carbon build up in cylinder, providing an ignition source.

Damaged/warn exhaust valve seat.

Sudden change (lean) in fuel mixture (covered above)

What you are experiencing is a symptom. Your backfire is probably caused by unburdened fuel/air getting in the muffler and firing off so I’d suspect a leaking exhaust valve.