This sounds a little odd to me ( bit of a poor joke there)...
I use a scarlett 18i8 but in mono...straight out of the main outs and it sounds really good. Conversely I've never had a great sound out of the headphones.
I think people have touched upon the possible issues but the outputs will sound different for different reasons ( different monitoring, no space setting etc).
I might be wrong but I suspect its not an issue with the interface and probably not a stereo vs mono thing....