A minor issue with the reference manual for the 1.5.0 firmware – nowhere in the manual (that I can find) does it state the source for the SQ-Drive Stereo recording. I’m assuming it’s the first 2 USB channels, but it would be good to be explicit about this.
You are correct – it is indeed USB channels 1&2 used for stereo recording and playback.
An extra point to this effect has been added on both sections in the reference guide.