-frequency response is just a tiny part of how headphones sound, it’s useful info but you can’t see whether headphones are good with just frequency response. -apple dongles are pretty good. -the jack on your pc/laptop will probably sound better than a super cheap amp. -EQ is fun and useful.
Learn to appreciate well mixed and mastered music, you’ll notice it the best on good headphones. I’ve completely rediscovered music after I switched from “normal stuff” to my hd560s. The downside is that when you know music can be that good, some of your favorite music won’t be nearly as good because it’s lacking in clarity and detail.