darkstar1
As you correctly said your dual 4 is 100kohm input impedance, which makes it a perfect match with the Schiit Freya in all modes.
As not only does it have a great impedance match, but if you look in your user manual you'll see it only needs 1.2v input for full wattage output, so it also has a very good input sensitivity for the Freya being low gain in all 3 modes. You don't need a preamp with high gain, you'll be using the volume control down near zero.
Cheers George