Buffer size / SQ

both, sometimes N2+ with onboard output jack. But usually with either a Khadas Tone Pro or a proJect Pre-Box S2 Digital. Sorry for going off-topic…

Sounds like a earth loop. feeding them through USB or sperate PSU?

yes, with the official HK PSU, no issue with an ultra low-noise. With the first Odroid N2 (not N2+) there was the same issue.