I too would like OBS to support surround sound recording.
Currently OBS cannot, as far as I can tell. My attempts produced an echoing stereo sound in the videos. Likely because it's just combining front and back channels. Turning off surround in the games fixes the echo, though still only stereo sound.
FRAPS is the only thing I've found which can record surround in software. Sadly it does not appear to be supported any more, and many games don't work with its recording capability. Hardware devices as you have could do it, though I've not found [any] with that feature.
EDIT: There are reports that Black Magic's Intensity can record surround sound:
https://www.blackmagicdesign.com/products/intensity