First - if you're satisfied with your listening experience, I will be the last person to tell you that you're not hearing something right. We all have different preferences, and if you're satisfied, that is THE most important factor.
That being said, I think your question is a good one. I decided to re-do an old comparison to better describe my thinking since audio memory isn't so accurate.
Using a few tracks from Qobuz, I compared:
iPhone > iPhone Dongle > Mega5EST
vs.
iPhone > iPhone Dongle > Sound Tiger Potenza > Mega5EST
The Potenza is a (powerful) IEM amp only with no DAC section... I didn't think comparing the iPhone dongle to another DAC / AMP with a more powerful amp stage (e.g., Mojo 2 or DC Elite) was particularly useful.
Conclusions:
When Mega5EST is driven by an amp, there is better dynamic contrast (loud vs. quiet passages), atmospherics (better rendering of high frequency details), more textured bass (grunt and rumble is more present and discernable). Using a good amplifier with the iPhone dongle and the Mega5EST improves my listening experience. This is not to say the Mega5EST sounded bad on the iPhone dongle - it still sounds great. I just believe it sounds better when powered with an amplifier.
Caveats:
- A/B comparisons are tough to do, especially volume matched. So take what I say with a grain of salt.
- The Potenza is most certainly adding its own flavor, and I do not believe it is 100% transparent relative to the source that feeds it. That said, I think the differences I'm hearing have more to do with the added power and less to do with the sonic flavor imparted by the amp.
- This conclusion also held true with the Dita Project M, but less so - the Project M still sounds wonderful directly powered by the Apple dongle. So much so that, for the sake of convenience, I use this combo the most when I'm on foot in the real world.
- All of this is useless though given my ultimate sonic preference when convenience does not matter - powering both IEMs with the Mojo 2 (without the Sound Tiger Potenza).