Yes, I have read quite a couple of comparison articles and reviews, and while I have found the same general comparisons as you have listed in your comment, there was one particular thread on Head-Fi that was very concerning. It was the DX90 vs X5 thread (or maybe it was the other way around), and what I found on there was a majority of people preferring the X5, for a couple reasons. It worried me a lot because a lot of people were saying that they preferred the X5, or just didn't like the DX90 and liked the X5 more when they tried it out, and some even said that they hated the DX90 enough to return it. However, I didn't see anything like that towards the X5, there was only praise for the X5. There are some people like you who say that the DX90 is superior to the X5 in terms of having a more neutral and accurate sound with a larger soundstage, but there are some very experienced users on Head-Fi that prefer the X5, and when somebody who is very experienced, uses a lot of high end equipment and obviously knows what they're doing says that the X5 sounds better to them, that's when I start to cringe at the fact that I have already placed my order in for the DX90.
In the end, it's all about your own preference. One terrible inconvenience that I have is that there are no audio stores in Arizona that carry either of these great DAPs for me to audition first. Also, I cannot really order both to try them out then return one because I don't have that kind of money to throw around, I'm a student.
While a lot of people like to pair up the DX90 with some good neutral sounding monitors, I would probably pair it up with something on the warmer side of neutral, if that makes any sense. Like a GR07 MKII, or GR07 MKIIBE is a little bit on the warm side of completely neutral. I have also heard some people say that the DX90 is slightly bright.
Shure makes some good IEMs, but IMHO there are some way better IEMs out there for the price. For example, the Shure SE425 is not worth the money at all, really. The Shure SE315 is also not that good for the amount of money you pay. I think that the only Shure IEMs that have a good price-to-value ratio are the beginner SE215, intermediate SE535, and the SE846 for those advanced listeners. Also, most of Shure's IEMs are dark and warm, not cold or neutral.
The IEM I am currently using a GR07 MKIIBE and the circumaural monitors I am using are the HiFiMAN HE-400. I also have a Mad Dog from Mr. Speakers on the way. I listen to a wide variety of music so, instead of listing all the things that I like to listen to, here's what I don't like: most Country, and all of techno/trance, though I do like EDM, which is different.
Lastly, can anyone direct me to a good source of downloading good quality recordings in FLAC or AAC format for free or a relatively low price? HD Tracks is eating all my money lol :)