Europa

IE600s are notoriously v-shaped IEMs that are pretty far out from neutral with exaggerated bass and treble. In terms of form factor there isn't really a like-for-like replacement for something as small as the IEs. Some additional recommendations worth checking out: Aful Dawn-X, laid-back and natural sound, almost inoffensive. Symphonium Crimson, aggressively fun like the IE600. 4 BAs make it one of the simplest constructions in that price range. Symphonium Europa, a more balanced brother to the Crimson. Letshuoer Mystic 8, very dry tonality but that's also what makes it special. A statement piece of an IEM. Actually close to neutral, as in, most people will find there's too little bass. Yu9 Que, if you could get your hands on one. The hype mainly comes from its price-performance ability and it could absolutely go toe-to-toe with many $1k sets, though I find its tuning stereotypical. It's a solid all-rounder choice if I don't know what your tuning prefs are. 64audio Tia Trio, if you're considering the U12t already you could also consider this. Some of the best bass I've heard in an IEM bar none. U12t sounds boring by comparison. Sony IER-M9, another laid-back oldie and a perennial competitor to the IE600. Depending on where you are, they could be had for cheap. Warm neutral presentation with treble boost, dynamics are its Achilles heel. Thieaudio Valhalla, much more refined v-shape presentation than the Monarch MkIV and more comfortable. Given your budget goes up to $2k, this is very much worth checking out. In terms of fit, Thieaudios are notorious for big shells and contentious comfort, so the advice is always to try before you buy if you could. You'd have the best luck with 64audio, maybe Symphonium, but Symphoniums will take some time getting used to because they aren't vented.

The DT 770 is a godawful disaster of a headphone. You didn't give a budget, so here's a bunch of IEMs that define their price bracket, at random prices: Tangzu Red Lion, Juzear Defiant, Yu9 QUE, Mysticraft Myth, Letshuoer Mystic 8, Symphonium Europa/Crimson/Zenith, ThieAudio Valhalla, Elysian Annihilator 2023, Subtonic STORM, Brise Fugaku. Maybe you didn't have them in your ears deep enough or didn't have a tip seal or something.

https://preview.redd.it/f18ckk4fi6pg1.jpeg?width=1440&format=pjpg&auto=webp&s=53aa1ff93152031ac11ef2bf16573aa39f29c88c $230 DT 770 Pro X vs a $23 IEM Music producers of all experience levels recommend horrendous headphones, (I produce music) and gamers (I am one) usually don't understand sound quality, and think there's headphones for gaming as opposed to headphones for music. I own $400 Yu9 QUE and $3,000 Elysian Annihilator 2023. I'll eventually buy the Subtonic STORM. I know what headphones are good. I had the $330 Martilo and it's really bad compared to the Yu9 QUE that replaced it. This graph isn't even a taste difference. The bass is extremely incoherent, and there's a huge peak in the lower treble. I owned original DT 770 (which is about the same) and the bass was so bad I knew it was bad before I became an audiophile. Apologies if I come off as rude.

I have the power of high-functioning autism. But yeah, I have high standards, and I have figured out what tends to sound good based off of graphs and descriptions, and I roughly know the kind of sound I like (bass boost of at least 7dB, normal mids, relaxed lower treble with a lot of upper treble extension and sparkle) I recommend the YouTuber Precogvision: https://youtu.be/8bm1oA5Xras?si=B7LJFATpvLDbZH66