On my desk at work I have a D90 connected to an A90 with short XLRs. The A90 was driving Beyerdynamics T1, which I modified to be balanced [4 pin XLR] or HD800. I'm often alone in the office so I also have a Rotel power amp driving KEF Q350 speakers.
My intention was to use the D90 as a fixed output level DAC, disabling its volume control, then use the A90 in preamp mode to control the volume level to the power amp via RCA. But that caused a hiss through the speakers. The hiss disappeared when I enabled volume control on the D90 and connected that directly to the power amp via RCA, limiting the A90 to headphones only.
Yes, it was disappointing because the A90's rotary control is easier than the volume up/down buttons on the D90, and I'd heard that disabling the D90's volume control [bypassing its preamp] improved its sound quality. The A90 is designed to be a preamp but didn't work well for me.
I haven't tried grounding the A90's RCA inputs. It's an interesting suggestion. I will try that when I get time but the A90 has been pushed aside since I found a cheap, used Trilogy 931 for use at work. The upside is H/P sound quality. The downside is that I now need 2 RCA outputs from the D90 [one for power amp, one for 931]. The D90+A90 stack was a neater solution.