Thanks for your response. I guess I prefer a more detailed sound, but with a full-bodied bass when listening to heavier rock music (which is where the E11K has helped). With my current setup I'm finding quieter passages, say involving acoustic guitar, not as crisp and defined as I'd like. I do strive to seek out older CD releases with higher dynamic range when at all possible. As I mentioned, I'm not sure if the X5i is the weakest link in my setup.
I do think that I'll need to move beyond my X5i at some point given the advances in DAC technology over the past few years, but I'm concerned that the X5iii may not be a dramatic enough a change to justify the price. I'd probably be better off going straight to the X7ii, since I've read that it excels in terms of detail. But I don't know if I can justify that price tag given that my overall setup is pretty mid-fi right now. Clearly the E11K wouldn't be necessary anymore if I went with the X7ii/AM3a bundle, but it seems like overkill for my $200 Fidelio X2 headphones at the moment. Then again, I guess I would be 'future proofing' myself for an eventual leap to HD-650s or something in that range.
Given what you mentioned about the X5i being neutral, whereas the X5iii is smoother, perhaps the X5iii is not what I'm looking for after all. Maybe I'll just move on to the X7ii eventually.
That being said, I think the X5i is still a wonderful DAP overall.