There's nothing to say on sound other than it is transparent. Perfect. Pure bliss. Very silent and very distortion free. A true high fi piece of gear at an outstanding quality. The bad news is you need eq to make it sound good as with all neutral gear.

You have now 3 slots for peq (parametric equalizer) in app that can be memorized with a preset. Unfortunately you can't name them so you have to remember which is which when you change headphones. UI as an experience is a little frustrating, there's nothing difficult, it just is not the smoothest of the experiences. Peq only works with peak filters (10 bands which is ok) and I get the feeling by ear that it accepts only integer gains (eg 0.2 becomes 0, but also 0.7 becomes 0. You might want to try and round them up..). The app is a work in progress so it might get better when you'll read this. The most practical solution: load your target for your headphones on autoeq.app (for the majority of people the standard target works just fine), select personalized peq and put in ten peak filters. Let autoeq do its magic while you take the quickest coffee of your life. Put the data gathered in this way inside the Fiio app. Boom. Great sound.

Great quality product, solidly recommended, it gives me a lot of pleasure.

Truthear Zero Red is a true Hi-Fi IEM in what may be the more classical definition of "high fidelity": it makes you feel as if you were live at the concert the moment you wear them and press play. These are really a true fidelity instrument and a very refined one, don't underestimate them because of the price: the tuning is, simply put, correct, you can hear all frequencies of the spectrum loud and clear. There are no annoying veils or bothersome muffling with this IEM, just pure joy and the sensation to have broken the barrier of reproduction to make a step in a live performance. Only a step though, as we get more analytical and take out our Sherlock Holmes magnifying lens we can hear that vocals are indeed a smudge thin and that in particular they feel empty as if they were an empty amphora; I try to explain myself better here: you have a correct lower end of vocals and you have a correct upper end of vocals, you just miss that very tiny accent in the central part of vocals. Things can seem pretty bad once one reads a review and that's because we don't understand at what scale we are talking. I am talking here in the micro scale, in the nano scale. It's just a tiny-iny little smudge in something perfect in that range. How tiny is this window in which vocals are lacking? And how much are they lacking. Ever so little. In fact just walking while you listen at these IEMS makes it impossible to detect this defect, I was only able to hear it while I was at home. On the go? No problem at all. This is probably due to the fact that zerored's tuning gives the accent on the presence region around 3 kHz, while you want to highlight 2 kHz to have a more natural presentation. 2 kHz is probably tuned a little lower to what would've been a full meaty vocal. Still a highly musical object in which female vocals shine particularly if we take out the tiny smudge mentioned above. Cymbals are also a little thin, very neutral, NOT lush, organic, 3D, realistic, shiny. Still very well resolved.You miss the metallic part of the sound and you get a bit of the *splash*/hiss typical of the neutral tuning. Bass (and in particular all sub bass) in terms of a natural presentation is a tiny smudge above what I'd like. I like a very natural presentation and I don't like a lot of bass. I like this quantity of bass on headphones, but on IEMs I feel the sub bass can't scatter properly and it's all directioned to my ear. It needs to scatter a bit, so I'd say I like a tiny smudge less. Still these IEMs go pretty close to my bass target, which is very difficult.

Eq works very well with these IEMs as they are almost a full Harman (so probably closer to your preferencies in every region of the spectrum) like Crinacle likes to shout when he's hanging around (at least this is how I picture him, LeMeme) and they have a very low distortion (you can see here https://www.audiosciencereview.com/...ruthear-x-crinacle-zero-red-iem-review.44865/ ), which means that they bear with eq very well on all the spectrum. With the Harman target by Oratory1990 horns become a more 3D experience. Still unfortunately cymbals sound thin. That might be probably due to the high frequencies region of the spectrum in which we are blind, analyzers pick up a lot of reflections in that area and data in this are is to be discarded. That means that probably a diy tweak might give some natural decay to cymbals.

Overall these IEMs sound resolving, clean, transparent, fast, as a true Hi-Fi item should be. Very realistic. I am worried about the idea of upgrading because they sound so good that the risk of buying something that costs 10 times as much that sounds worse is very real. As I am typing it may make more sense upgrading from these only going in the 1000 dollars zone and above. Yes, they sound so good.

Fully approved and three thumbs up for the Truthear Zero Red that score A- on my rating.

Appendix: Explaining of my rating:
-Anything beneath D: defective item
-D : as in decent
-C : good (most consumer items land here)
-B : very good
-A : perfection
-S : WOW (took my breath away)
My ratings do not consider price as a factor and are purely and solely based on sound quality.

Frequency response graph courtesy of Headphones.com