[1] Then I converted the 24/192 to 16/44.1, and for some reason, it turned out "worse", upper frequencies cut, less information, and it weighs less than the original 16/44.1 that I downloaded.
[1a] I know that more high frequencies doesn't mean better quality,
[1b] I just don't want to lose the airyness of some songs,
[1c] so I want to keep as much information as possible.
1. Why is it "worse" to have "
upper frequencies cut, less information, and it weights less"? I don't know what you mean by "weights less" but "upper freqs cut" and "less information" isn't worse, it's better!!! Because of the following points:
1a. Correct. In fact, more high freqs can often mean worse quality as it's liable to cause distortion (IMD).
1b. "Airyness" exists in roughly the 8kHz - 16kHz range and mainly between about 10kHz-14kHz. Therefore:
1c. Why would you want to keep information significantly above that range?
[1] I mean, the fact that the driver of my earphone is trying to make that sound does affect the overall sound,
[1a] if you understand what I mean.
[2] If you record a pluck of a guitar, and then cut out all sounds under -70 it will sound very different to the unedited version.
[2a] So I just want to keep as much info as possible.
[3] I can't just download m4a files, because they are usually mastered for iTunes and sound funky.
1. But that's NOT a fact, the driver of your earphone is NOT trying to make "that sound"! For example, let's say you have sounds down at -80dB, the noise floor of the recording is say -60dB and you're listening at a peak level of 70dBSPL, your earphones are not trying to make those sounds down at -80dB, they're just making noise that does NOT affect the overall sound.
1a. Not really. Exactly what the driver of your earphone is "trying to make" is dependant on a number of factors. Additionally, A. "Trying to make" and actually making are not necessarily the same thing (IMD for example) and B. Even if they succeed and the overall sound is affected, that's no guarantee that the difference is in any way audible.
2. That is simply NOT true. If one were to cut all sound below -70dB in the vast majority of reasonable reproduction scenarios it would sound absolutely no different whatsoever, let alone "very" different! Even in some fairly extreme (but still reasonable) reproduction scenarios where the difference *might* be audible, it would still only sound slightly different, not very different.
2a. Again, why? What benefit do you think you gain from keeping info that's inaudible?
3. As the name suggests, "Mastered for iTunes" (MFiT) means mastered specifically for AAC 256vbr. Assuming you download the original AAC 256vbr file, you are getting an exact bit perfect copy of what the mastering engineer released/intended. So, unless you have some serious fault with your equipment, if it "sounds funky" then it's supposed to sound funky and why would you want to change "funky" music into something else?
It's pretty clear that you don't really understand what you're looking at with a spectrogram and how it relates to either what your earphones are "trying to make" or what is actually audible, as others have effectively stated. Furthermore, as
@pinnahertz stated, unless you are doing your "experimenting" using ABX testing (at reasonable playback levels), then all you are accomplishing is the creation of a "strong perception bias" and FALSE results, that completely void your experiments!
G