Sound quality is fine with WASAPI, but I have a couple minor issues. It always drops some samples from the beginning of the first track played and is unstable with Milkdrop (frequently crashes/locks up Musicbee when starting the visualizer).
So I decided to mess around with it a bit more. Turns out the multi-channel problem is definitely ASIO4ALL. Or maybe ASIO4ALL in combination with Intel HDMI audio.
Anyway, I tried FlexASIO with the kernel streaming backend and it works perfectly. No multi-channel issues, no dropped samples at the beginning of a track and no crashes with Milkdrop.
Just thought I'd pass the info along in case anyone else has similar problems. If you're looking for bit-perfect playback with a multi-channel config and you're having issues with WASPI exclusive, give FlexASIO a shot.