The JBL Reference 420 was my first "real"
headphone. I don' t know whether I 've been too
used to its sound signature, but I just ditched an Audio Technica M50 for the JBL .

Bass: powerful with lots of extension. From other reviews of JBL 420 most of them saying the bass is bloated & boomy. Well this happens IF you put it on your head in the wrong angle. You have angle the headband forward, and position the earcup to the back. This way you will get the best of the JBL , no bloated & boomy bass, but with powerful mids and crystal clear highs.

The mids are outstanding, making M50 sounded weak & thin in comparison.
The highs reproduction between the two is very different ,which I found a little bit weird. I have the Hifiman RE-0 , which is the most natural sounding &
brightest most detailed earphones I've ever heard.. and the highs reproduction of JBL 420 sounds similar, so i guess the JBL is more accurate .

Soundstage: M50 thrashes the JBL.
Isolation : even though M50 is the bigger
headphone, but the JBL isolate outside sound better. Comfort: JBL 420 takes the home run here.. I 've used the cans > 4 hours & my ears only got a little warm. With M50 I used it for an hour & my ears already felt tired & itchy. The JBL is VERY comfortable to use, even while napping on your bed.

Pros: * When positioned well: JBL 420 sound quality bests M50 ,especially in the mids where M50 's EQ is V - shaped (recessed mids).
* Very comfortable to use over very long duration
* Isolate very well

Cons: * When you use it at the wrong angle, it
become a bloated & boomy bass cans where the mids & highs are overlapped by the too strong lows. * Soundstage could be better
* Carrying case is ridiculously huge!