I received the ADI-2 DAC yesterday and I'm in love. I want to say thanks to amirm for the measurements and review putting this on my radar as I was actually looking at pairing a Topping D70 with an Atom and THX789 but switched to this. The measurements simply let me know I was getting an excellently-engineered DAC/Headphone amp and the strong feature-set put it over the top. The size and build quality is great with attention to detail like the locking power adapter cable and slow ramp up of volume when switching outputs for safety.
Attempting to answer your three questions
- I use mine with USB (Main PC), SPDIF (Laptop) and Coaxial (Chromecast Audio). After I found out I needed to install the driver to update the firmware, and getting the auto-sample rate changes in Tidal situated with ASIO Bridge, it's been smooth sailing.
- No idea about DirectSound, sorry.
- The RCA outputs have full volume control. Just tested with my studio monitors.
I too wanted to EQ my headphones. I love my HD600 but they need some sub-bass help and this DAC not only makes it easy but the second you unplug them, it switches back to flat for my studio monitors unlike a software based solution.
The RME was flat out above what I planned on spending for all of this but I'm more than happy I did it.
Notice I said nothing about the sound. I think you can get comparable or better sound for cheaper, it's the features that make this unit a standout.