The Club in A is a good option, if a bit of a walk. For some B gates it may be faster to take the train to S terminal and go to The Club location there. Then complete the train loop back around to B when it is time to depart. That's if the S location is open, though, since it closes at 5:30p.
With AS kicking PP users out of its SEA lounges as of 9/1, having both as options may come in handy if overcrowding becomes an issue.