So there are several differences. 851n is digital only preamp, without any analog inputs, where now also has analog. Build quality is much higher with nq as would be expected at triple the price. 851n uses analog devices dacs where nq uses ess dac. Remote with nq much higher quality.
As for sonics, just more of everything with nq. But they have similar sonic signature. I think you’d be happy with either. The 851n definitely the better value