"Hmm: I'd better check if Poser 'traps' imported OBJ+MTL ambience settings >1 as 'invalid'..."

Set Poser primitive 'box' super-ambience as eg 10. Exports to OBJ+MTL with Ka 9.9  9.9  9.9. Re-imports as Ka 0 0 0

Seems it does...

{ Yet more caffeine required... }