Sample:
The swing of his nature took him from extreme languor to devouring energy; and as I knew well, he was never so truly formidable as when, for days on end, he had been lounging in his armchair amid his improvisations and his black-letter editions.
The sentence comes from the Red-Headed Leagues. Watson describes both sides of Holme's characters: artistic vs. astute.
When Watson describes him as "he was never so truly formidable as when,....", does he mean that Holmes is not formidable at all when he relaxes at home, or that even lounging at his armchair, Homles is truly formidable for those criminals?
It looks that "be never so truly formidable when" is different from "be never so truly formidable as when".
Does the sentence "be never so .... as when" indicate negative meaning or it's just a way to emphasis?