Bummer, PC38x is one of my favorite headphones period. Except for a driver going bad and having to buy a whole new pair because Drop doesn’t sell parts… People can hate on “gaming” things all they want, but sometimes it’s just more convenient to have the mic attached and if the headphone also sounds great, why fight it?
You are partially right. IEM stands for in-eat monitor and that name comes from the fact that it is like having a monitor (like a stage wedge) in your ear. That is all that means. It doesn’t have to be used on stage, it doesn’t have to hook up yo ang particular transmitter/receiver, it’s just an audio playback device that fits inside your ear canal. It is no longer restricted to musicians, it refers to a huge array of consumer devices, too. Nearly all of them have a standard 3.5 mm plug on the end, though you can probably find some cables with larger balanced plugs.
Headphones are worn on your head, so they have a headband and sit on or around your ears. Ear buds (and probably earphones, though I don’t see that term much anymore) are like regular Airpods that go in your ear, but they don’t get shoved down into your ear canal and they don’t create any sort of seal.
The only difference in tech is that smaller devices require smaller drivers (the “speaker”), so they may or may not use the same kind of drivers. But all of them serve the basic function of moving air to create sound waves.
In terms of FR, the HD560s are quite good, but I feel like they compress the dynamics a bit. Probably wouldn’t be the only pair I use (but I’d always check on multiple anyway), but they are hard to beat in terms of price vs performance. The Beyers can be good for treble analysis in particular, but I have a big head and they are just not comfortable for me. Still, those would be my two picks.