What SQ value does a high end stereo preamp add to a network player?

If the Lumin X1 is your only source,  Lumin’s Leedh Processing 
should be fine . No need for a pre amp.

´´can i use x1 as a pass thru from ht processor for mains?´´  : no

But you could use something like that , from the Lumin to the power amp. and from the AV processor to the power amp.https://goldpt.com/sw2x.html

Then the quality of the power amp is important