It really depends on what part of Portland you're in. Yes, there isn't a huge Seahawks bar, or rabid 12th man prescence, but there are a LOT of Seahawk fans here in Portland. Portland prides itself on being "weird' and liberal, so it's not conducive to a huge football culture, but they are rabid Ducks and Beavers fans, and I've seens tons of people rocking Hawk gear.
Try any of the Blitz bars, there are 3 or 4. They are a sports bar, so they have all the games on at once, but this will be the only game, so there won't be a bad seat in the house and they have tons of big screens in high def.
Most bars I've been to play Hawks games, and they usually turn the sound up for me.