I'll be in Portland in the early spring and would love to check out the music scene--I'm a musician myself so there's not a kind of music I don't enjoy. What's the one or two go-to places in town for a low-key night of good music?
Great choice, John - you're coming to a place rich with music in a variety of styles and traditions! There's typically a festival of some sort somewhere in the area most any time of year.
A good place to start is by perusing Willamette Weeks' events postings: http://www.wweek.com/portland/flex-202-music.html
I'm a blues fan, so offer this great link for events and venues calendar: http://cascadebluesassociation.org/blues-events/calendar/
Most everyone gets to Jimmy Mak's downtown. Duff's Garage Bar & Grill is another good spot. McMenamin's Kennedy School is a unique venue - bars and restaurants in an old school, with mostly weekend performances.
While you're here, you might want to tune into KHMD (89.1) for jazz and blues as you roll.
Mentioned in this answer: