I can't comment on the S2 but the S1 was crap compared to N900. One of the reasons I ditched it after 1month. Using Sennheiser and shu headphones. The GalaxyTab also sounds poor compared to N900 and so did the OmniaHD. I think audioqual is not one of Samsung's strengths.
But again, I have not had time to try an S2 yet.