@CaptainFantastic, I have both. I recently purchased the RME ADI-2 dac specifically because my IEMs (andromeda and xelento) would hiss on the grace m900 and I also wanted balanced pre-amp output. They both use the same AKM4490 dac chip. I've only had the RME ADI-2 for a day though. To be perfectly honest but also very unhelpful, I personally don't think there are very big of sonic differences between the majority of dacs despite what it reads on head-fi. I have done tons of direct comparisons of different dacs with TOTL headphones, and it is extremely difficult for me to pick out which dac I am listening to in a blinded volume-matched comparison, unless the dac actually sucks. When comparing two good dacs, if there are any sonic differences, they are inconsequential to me, as I have tried and failed to consistently identify which dac is which when blinded, and I believe that good dacs should be transparent so there should not be any significant additional coloring that would allow me to tell which dac is which. If you have a different personal experience or perspective, then I think my perspective will not be helpful for you. The ADI-2 measured extremely well by ASR, so should be basically transparent. I am sure the m900 would also measure very well though. Subjectively, the ADI-2 definitely sounds really good. I did a quick side-by-side comparison just now. For my HD800S, very rough approximately volume levels, I am around the -25 to -20.0 dB low power mode volume setting on the ADI-2 for comfortable listening levels and on the grace m900 I usually go to 60-65. Personally, if the m9xx had balanced outputs and black background with IEMs, I would not have upgraded, and I always believe my money goes further when spent in my headphone/IEM/speaker budget, so I would recommend If you are already happy with the m900 to just stay with it, but if you would benefit from the features of the ADI-2 dac, it is an extremely good dac/amp. If you had any more specific questions, I can try to be more helpful.