I thought I’d reply in the tinkering section 
Assuming you don’t want to change DAC of course:
From experience (not expertise), having had a very similar chain to you (including FMC’s, USBridge and S2 DAC)… the best ‘bang for the buck’ won’t come upstream of the fiber optic cable…
Something a little over the budget of the NUC that helped the S2 DAC performance for me, was upgrading the USBridge to something like an ultraRendu…
Something well below the budget of the NUC (and given the USBridge is already very good) is what I did with my FMC’s… I had SOtM upgrade my FMC’s. They replaced the switching regulators for low noise linear regulators and upgraded some capacitors. This also allowed the FMC’s to run on 5Vdc (and you know what that means…).
This FMC upgrade cost under $150 from memory and helped… to my ears of course.