The guys over at Sounder at Heart have done a good job of pointing out why most of the talk of the Sounders being up a creek in the second leg is bunk. Here's one gem regarding the "only 2 teams have won the two leg playoff after dropping the first leg at home" statistic:
Letting that number stand alone is just plain dumb, though. For one, it ignores the total sample size: Only six teams (not including the Sounders) have ever even lost the first leg at home. Looking just at those numbers, we're talking about a whopping one-third of teams coming from behind to win, hardly an insurmountable task.
Looking a little deeper, we see three of those home-leg losers lost by more than one goal. Of the three that lost by one goal, two of them have come back to win. So two-thirds of teams that lost by one goal at home, managed to advance anyway.