I wrote a brief comparison a few pages back, but I will try to summarize my impressions. I know you are mainly concerned with SQ, so I will touch on that first followed by some other aspects, all of which are relative to one another (DX150 with Amp 6 vs N5ii, both balanced outputs)
Sound-wise, they are quite different. The N5ii is brighter, more analytical and has an edge in imaging and soundstage, due to a bit crisper treble. This can be a good thing or a bad thing depending on your preference and phones. With Andro, I felt the N5ii was amazing as far as soundstage, airiness and separation but it could become fatiguing after a while. The DX150 to me is more like a nice tube amp - a touch warmer, great bass response and a somewhat silky treble which makes it a good pairing with the Andro for me. Soundstage is still good but not as precise as the N5ii and the same with detail retrieval - again, this makes the listening experience less fatiguing and more enjoyable overall. The N5ii has a blacker background, especially on Low or Medium gain (I never touched High with Andro). With the DX150 there is some audible hissing with Andro even on Low gain, as well as some audible pops when switching tracks - worse on High gain but still there on Low. As I said earlier, I think the N5ii is untouchable for the sound quality you get at the price point. I would like to try the DX150 with some of the other amp cards at some point, especially the 4/4s if I ever have a chance to get my hands on one.
So, if the N5ii is arguably better or at the very least as good as the DX150 (albeit different) in sound quality - what makes the DX150 better overall to me? In short, everything else. Yes, the DX150 is noticeably larger and heavier than the N5ii. This is not a deal breaker for me personally, and the build quality of the DX150 is superb. Battery life with the DX150 is better in my experience. The touch screen of the DX150 blows the N5ii out of the water - the color accuracy and contrast is worlds better. The volume knob is better with a more tactile feel and feedback, although it is a touch easy to turn so be careful if you carry it in your pockets. UI is very responsive and snappy with the DX150. Yes, there is zero documentation that comes with the DX150 out of the box. Yes, I struggled to figure out what I was doing with it initially, but once I did it has been flawless for me. Scanning in my SD card is quick (N5ii may actually be faster) and I have had no problems. I have not tried to connect the DX150 to my MBP directly so I cannot comment on
@jeffhawke 's issues there - I always put my SD card in my laptop to copy over music/firmware updates, etc. The fact that DX150s are not shipping with the latest firmware is not surprising nor should it realistically be a detractor for most people. The manufacturing and distribution process would not necessarily allow for them to go back and update devices that had already been assembled, QC'd and packaged for shipping - or possibly were already in a warehouse ready to ship. The firmware update process was quick and painless for me. Using the DX150 as an external DAC on my work computer (Windows 10) was a bit of a trick to set up initially - I wound up downloading the driver for the DX200 and it has worked well since - audio quality is perfect with no clicking or noise - but I did notice that the DX150 battery drains incredible fast when doing this.
Sorry, I know that is kind of all over the place but those are my thoughts on the two players. They're both great, both have their own strengths and weaknesses and ultimately I think it comes down to user preferences and priorities.
