Your recommendations for cost-effective and musical sounding headphones?

I could say that common sense standard is not to spend less than needed. The difference between Grado 225e and RS-1 is big even when just listening to youtube without external dac/amp. $500 is next to nothing these days. But for most people spending more than $100 would be madness or at least weirdness. Or even more than $50.

I've used Grado RS 2, AKG 702's, 240's and Senn 600, as well as many others. IMO, Hifiman 400's kills all of them and have been the biggest and most significant upgrade in sound that I have experienced in a long time.

Great comments here. I need to hear HiFi Man and Audeze to get a sense of what alternate designs contribute to headphone listening. Recently bought a JDS O2 amp for my Senn Momentums and had no idea they could sound this good. Clearly amplification also makes a difference in perceived headphone quality.

This Phillips set compares favorably with the Senn 650 (though I prefer Senn 600 myself)  for about half the price:

https://www.amazon.com/Philips-X2-27-Fidelio-Headphones/dp/B00O2Y2MZG/ref=sr_1_1?ie=UTF8&qid=146...