Spent my college years and early 20's in Seattle helping throw parties at a variety of underground/DIY-ish venues (including ETG, two other loft spaces down Pike/Pine from ETG, and The Monkey Loft back when it was literally Brian's loft with his bedroom in it before he remodeled it and turned it into a licensed space). There's definitely a stark difference in vibe compared to larger and/or legal venues, and even a noticeable contrast between The Monkey Loft's early years vs. it's legal years.
Something about those spaces feel more organic, more raw, more unfiltered. Everyone is more connected, the artists seem to have a much better time, the crowds are often more diverse and you'll hardly ever see someone staring into their phones taking videos. That doesn't mean that good shows don't exist in larger or legitimate venues, just that I'm often very discerning with which large venues I'm willing to go to.
I'm going to somewhat tangent this to pick your brain:
Transplant to seattle from Baltimore, back east I knew all the good venues, DIY and otherwise, and had enough connections to friends working radio stations/djing gigs to stay in the loop. Out here I'm utterly clueless, even to something as simple as the basement EDM scene. Any tips, if that doesn't violate some unspoken dogma? :P (hip hop, electro, doom/grunge, punk, whatever.)
I did move away eight years ago, and while I keep strong ties to the people in that scene and still have a good sense of what's going on, I either don't know of the truly DIY spots right now, or there just haven't been many for the past few years. There are some quality legal underground spots, but my knowledge of that is limited to dance music.
Kremwerk is probably most like what you're looking for, in that it's a small, late-night basement that throws great parties with fantastic music. The Monkey Loft would be your next bet, though per my original post, some if its luster has faded since it remodeled itself and became legit (but they still bring great acts). Rebar also gets an honorable mention, and while I wouldn't say it's too "underground" of a venue anymore, it's got a lot of history as such (read into it!) and hosts the longest-running house music night on the west coast, Flammable (seriously, check it out, especially if my man Eugene Fauntleroy is playing). Last recommendation for a venue would be to check out the next Train Car House Party[0].
Your best bet would be to follow area promoters. Sweatbox and Shameless are long-standing groups from Seattle with a history of throwing great parties. Sweatbox doesn't throw as many parties as they used to, but just last month they threw one in some kind of garage space in SoDo with Derek Plaslaiko playing. I worked with both crews when I lived there and they're wonderful people.
Other honorable promoter mentions are Innerflight, Hunt & Gather, Grounded, Drop, Viva and Depth. While you're at it, look into anybody involved with the Cascadia Festival[1] coming up, as many of the acts/groups involved throw small underground parties in the city. Decibel Festival obviously gets a nod, but they've been kind of quiet in Seattle since Sean moved to LA a couple of years ago. All of these promoters have Facebook pages that should be easy to find.
Wanted to write a very specific thanks, you went so out of your way with this and it's got me excited again, I've been far too down on the scene out here for the last few years! This absolutely helps.
I can't speak to Seattle, but in Oakland at least most of the shows (somewhat surprisingly) end up organized on Facebook. Once you know a band or two you don't have to dig too deep to find out when/where shows are happening.
According to my friend from there, the DIY scene has migrated to the areas north of Seattle proper. I don't remember any specific places unfortunately.
Something about those spaces feel more organic, more raw, more unfiltered. Everyone is more connected, the artists seem to have a much better time, the crowds are often more diverse and you'll hardly ever see someone staring into their phones taking videos. That doesn't mean that good shows don't exist in larger or legitimate venues, just that I'm often very discerning with which large venues I'm willing to go to.
/shrug