I don't think it's unreasonable. We all have our preferences. The RE-400 is a very competent IEM despite its low price. And if the sound sig fits him just right, then I can see how it would be preferable to a far more expensive IEM that has a different sound sig.
It's like someone preferring a "cheap" Sennheiser HD598 to a Grado PS1000, or a VMODA M80 to an Ultrasone Edition 10, etc. Just because the expensive models cost more, and probably do certain things far better than the cheap models, doesn't mean everyone will like their sound. Nothing wrong with that.
There are so many things I can say here, but I will just say that either there is now something wrong with AstralStorm's SE5, or our ears really don't agree. This seems to be the case of one person's measurements, and now ears, disagreeing with everyone else that I know of. If there are other people with the SE5 that feel the same as AS, I haven't found them. Measurements can be misleading and are limited, plus the human ear is an extremely sensitive instrument that, from my knowledge, no measurement equipment can compare with for analog audio signals. What is the true measure of performance for analog audio? How do you measure resolution and "proper" soundstaging? The human ear is a highly specialized, finely tuned instrument that can be trained as can be seen here and here.
I can understand someone having sound signature preferences and even changing their opinion based on comparing with other headphones. I also wanted to bring up that in my experience all bets are off with performance when someone starts EQing everything. EQing throws so many unknowns into the equation and unless someone uses the same EQ circuit (or software) along with the setting, repeatability becomes difficult. I can see why a multi-BA CIEM with crossovers may not perform as well as a single dynamic when EQed, and I have heard a lot of EQs that sound very unnatural to my ears. I understand there could possibly be ulterior motives, but I won't even speculate. For those long time members that are familiar with several other threads of mine, this reminds me of many posts by Pianist.
I hope ace8888 chimes in, as I am sure he will, once he has some time with the SE5. There does seem to be a consensus about the SE5, and statistically that makes more sense than putting too much stock in one outlier. There aren't many other places that talk about the SE5, but there are two here and here where you may want to ask questions.