I have been using the Sony XBA-Z5 IEMs for a few years and very much enjoy the sound. Prior to the DX-200, I was using the AR-M2. It drove these IEMs very well and provided strong, deep and well controlled bass as well as clear and well defined mids and highs. When I got the DX-200 with AMP1, I was a bit disappointed that the bass was not as deep or well controlled as with the AR. I just got AMP5 and now I think the DX-200 matches the AR.
The AR is a fine DAP, but the firmware support is lacking. It took over 2 years from initial introduction to the point when AR finally released decent player software that actually allowed you to scroll through by artist name and then select an album by the selected artist. With AMP5 I now feel that I have equal or better sound quality and far superior support.
Also, before anyone suggests that I try balanced cables and AMP3 or 4, please note that I use my IEMs for air travel and frequently plug them into my computer or airplane audio systems to watch movies. I don't want to switch back and forth between SE and balanced cables. Thus, AMP5 with its superior SE sound quality is the best solution for me.