I'll echo the vote for Echo. Kurt is both nice and knowledgeable. Fred's (on Hawthorne) has the vibe of a vintage stereo shop (circa 1960s) but a more limited selection compared to Stereotypes. Fred's is an authorized dealer for many high-end brands, but oftentimes doesn't stock "uncommon" components: Stereotypes does. Echo has the best used equipment, followed by Fred's. Regardless, all 3 are well worth a visit.
No audiophile trip to Portland is complete without a visit to Music Millenium on Burnside: it's a Portland institution. There are several other records stores of note scattered throughout the city and I'd not overlook the chain, Everyday Music, as that store (on Sandy) has a huge selection of used CDs (whereas MM has a better vinyl selection).