As an owner of the Dual Core it's good to get some "real world" feedback on the X4.
Flat frequency response is revelation, isn't it?
Have you tried using the Dual Core and DAC3 together (Dual Core for DSP and DAC3 for D>A conversion) and compared to the X4?
Last year had Schiit Freya and Gungnir MB in house. Rolled tubes on the Freya, tried all three settings, tried every possible combination with and without the Dual Core, ended up sending them back (after blowing one of the nice NOS tubes). They made 10% of my iTunes ripped CD library sound better but 30% sound worse. Note that I'm retired so have 40+ years at this and can put in lots of listening hours. Really wanted to like the Schiit stuff as I like the upgradability and no nonsense product design.