[quote]"You don't get your Unnatural Toughness & Strength simply because you aren't in power armor at the moment."[/quote]

They should still get it. Unnatural toughness and strength (x2) are TRAITS that are the result of their genetic/biological enhancements that occurred during the process of becoming space marines. The only thing power armor affects is a +20 bonus to strength and when in it you treat movement speed as though it were 1 step higher. 

Marines (or any creature with an unnatural trait) would still benefit from their unnatural traits regardless of what they are wearing.