Snake oil is a business term for something that is a fraud. So basically, don’t waste your money on expensive cables expecting better sound (yes, this is actually a thing in the audio community)

1. Use EQ. Just do it. It’s free and can make a headphone that is tuned bad out of the box, but has great technical abilities and other qualities going for it, into a real powerhouse (e. g. DT 770 80 Ohm). I’ve been looking for a good tuned closed-back that does not need EQ to sound good (I like Harman with a few tweaks), but I have failed to find one for me in the price ranges I was looking and I doubt there is a good one even now (disregarding the DCA Stealth) and with EQ my DT 770, that was way too bright for me in the beginning, is now my go-to closed-back since it’s comfortable, has very good technical ablities and soundstage, and with EQ I could fix the one thing that was bothering me: it’s tuning. without spending a dime.
2. Also, don’t listen too loud even if it’s fun and “sounds better”! You don’t want to be deaf when reaching 30 years of age.
3. AMPs and DACs don’t have to be expensive to be good. Even some USB-Dongles can drive kilobuck headphones just fine. Maybe you don’t even need and AMP / DAC if your motherboard has good onboard sound.
4. Cables are snake oil

While that is true, the performance increase is also quite noticeable and tbh, I think headphones under 100€ are very rarely worth it considering IEMs pretty much beat them in every category in that budget range. I think the first headphones that are worth it in their budget price range are the K361 and 371, the DT770 with EQ and the HD560S. Any lower than that and you basically could have gotten an IEM that does everything better except soundstage.

Not if they are on sale and they are much better.

Nah, not worth it considering the HD560S exists

Very bad