I second a modern Stern. EHOH is a great one.
They have a different character in play and rules to give you a more diverse lineup. A sys 11 isn't substantially different than your current Williams machines. JP, IMDN, DP would be great additions too.
People always laugh at band machines, but there isn't anything better than cranking up a Led, Aero, or ACDC really loud and knocking out a few games and beers with friends.
Or go the other direction and get a good EM.