The Showbox may be too small to hold the Seahawks or Mariners, but it's just the right size for the touring bands which play this medium-sized club (capacity 1,137) nearly every night of the week. Frankly speaking, it's fortunate that you love this terrific room as much as you do, because if you like live music at all, the Showbox is pretty much impossible to avoid.
You also mentioned...
Neumo's, Mirabeau Room, Studio Seven, Jazz Alley, Liquid Lounge, RockSport Bar and Grill, ToST, Fenix Underground and the Premier.