I used a Cal Audio Labs Sigma-II DAC for 10 years with my system.
When I got the 990 last year and used it in Upsample mode, I no longer needed to use the Sigma-II DAC.
I did some listening comparisons, and the 990 in Upsample sounded better than using the DAC.
So I sold my Sigma-II on eBay a few months ago.