For getting Dolby Headphone with the Vali amplifier.
Get an Asus Xonar DG, DGX, DX, D1 sound card, they all come with Dolby Headphone and a DAC chip.
Personally, I would get the Xonar DX, it's PCI-E and comes with a CS4398 DAC chip :)
Plug the Vali into the line-output/headphone jack on one of those cards, plug DT990s into the Vali.
You will then get Dolby Headphone from the DT990s.