This may be a subjective question guided anecdotally, but under which conditions can you start to discern true differences in equipment?
For example, I have Shure SE215s and SE425s, and typically, other than bass level, and slighlty clearer voice, I don’t typically discern much musical difference on my iPhone despite the cost and build differential.
Would I need to use an amp to get more from my SE425s? Improve musical compression of songs? Or is the 215-to-425 not enough to capture the difference and I’d need to improve my 425s to get large improvements? Is it IEM vs over-ear?
I am going to use my standard response here. Whatever makes you happiest, then that is the place for you. Essentially, you are listening to music in your head. In my never humble opinion, a cellphone is not a great starting place for audiophile sound And if your going to carry round an external DAC/Amp, Just get a DAP.
The biggest audible difference between the SE215 and the SE425 is in their tonal profiles. The SE425 is more neutral than the SE215. You’re already picking up on that from your comment above, so the first question is … what other differences are you expecting to hear?
The SE425 is somewhat more resolving than the SE215, but it can’t render something that isn’t there in the source. If you know what to listen for, which comes with practice, and have suitable content, then you should be able to pick up on those differences with some careful, focused, listening; such things can be very subtle.
The SE425 is sufficiently sensitive, and low in impedance, that adding an amp is really only useful for the purposes of getting a better impedance match with the output from your phone (depending on exactly which model it is).
Even if your iPhone has a 3.5mm output on it already, you might find the $9 Lighting-to-3.5mm adapter works better with the SE425 as it has much lower output impedance than the phone itself. That’s generally a good thing and is generally quite audible, with there being multiple potential benefits.
Other DAC/amp combinations will also impact the sound in different ways, sometimes increasing resolution, but in many cases the “improvement” comes simply from the relative impedance of the amp/IEMs (or headphones) being more appropriate.
But in this case, it’s not the SE425 I’d be looking at first.
I have a pair of SE425s. My iPhone, Droid and iPod Nano (1st generation) drive them with authority. My iPod Nano sounds the best. When you use IEMs make sure you have a good seal. A poor seal will effect the sound signature / quality.
Personally I am quite satisfied with my pair. I use it exclusively for mobile applications, so casual listening (MP3s). If I were you and would want to improve your mobile kit, I would look into higher end IEMs and a DAP first, then maybe a portable amp. Uncompressed vs. compressed might factor in, but frankly the quality of the master will have a much greater impact than either. Just my 2 cents.
So I don’t have any experience with those IEMs in questions, but I wanted to speak more broadly about the thread topic of discerning subtlety, which is a really great point of consideration. When talking about hearing subtle differences in gear there inevitably ends up being a lot of debate about objective vs subjective arguments and superiority of one’s gear. But I feel like people don’t talk enough about the actual skill of critical listening, which I think is crucial in hearing subtle differences. Yes, it is a skill that can be learned and practised.
Dr. Sean Olive of Harman put out a great piece of free software to help people train their critical listening abilities called “How to Listen”. These are often listening skills that most new people to the hobby don’t consciously think about even if they already subconsciously listen for these characteristics. The software is a great exercise in testing your own abilities.
I always feel that as people gain more experience, hear more gear, and listen critically to well mastered material they are intimately familiar with, hearing subtle differences becomes more apparent.