I've had a California Audio Labs Sigma-II Tube DAC for about 8 years until I got the 990 2 years ago. I did a direct comparison between using the Cal DAC + 990 in Analog Bypass and just the 990 in Digital Coax Upsample.
Between the two, I like the 990 in Upsample more than using the Cal DAC.
I ended up selling the DAC.