Just an update on this.
I got a firmware update from SMSL that fixed the DSD512 issue.
So I can recommended setting HQPlayer output to fixed DSD512x44.1k with SMSL D-6 (not DSD512x48k)
It works fine in native DSD512 with lowlatency Ubuntu 5.15 kernel - and fine with NAA OS image
This DAC is an absolute bargain if you want to experience direct DSD to analogue conversion with HQPlayer, with balanced outputs !