Mike, have you experimented with other ground locations on the 18NS? I believe the phono boards are completely decoupled from the preamp circuit when not selected, so grounding at the phono board won’t make a difference on other inputs.if i had to guess as to the reason for this; it's likely related to grounding in one way or another....and is not power supply related. since i always run my 18NS in battery mode. and maybe it's just grounding the separate charging box and that a better power cord matters for that.
i do have my Tripoint Elite ground box grounded to my 18NS (at the phono ground post) with a Thor grounding cable.