Considering I was in the same position not too long ago not sure about these headphones, here is my opinion on them (+ ATH-M50 comparison).
My use will be monitoring/surgery mixing of some elements, in conjunction with my Yamaha MSP5s monitors.
As soon as I pulled them out I thought there was nothing in the box, but they were just that light.
Of course I turned on a few records that I knew. I knew exactly how they sounded due to extensive listening/mixing in the past.
I was amazed by very linear frequency response, with only slightly lacking
low frequency spectrum (which is not a bad thing, considering most of the time headphones are bumped in the lows). I would say 2-3 decibel low frequency difference deficiency compared to my monitors.
Now back to listening.
The song/instrument separation was pretty amazing, where I could hear every element separated (in a good way), with no apparent bumps in the high frequencies (major issue I had with most headphones in the same price range).
The headphones were almost boring
ONLY because they are so linear, it's ridiculous!
Anything that I have mixed in the past and messed up was dead obvious here, no doubts, just thoughts of remixing my mistakes.
These are definitely NOT consumer headphones, they are perfect for finding the perfect instrument balance, and not being confused for a good mix where the mistake resides. Much more accurate than AKG 240s, ATH-M50, Sennheiser HD280s, and other headphones in this price range.
Now comparison of
Equation Audio RP-21 vs Audio Technica ATH-M50 (coiled cord)
ATH-M50 was generally much more comfortable set of headphones with a soft headband at the top.
ATH-M50 provide much better sound isolation, so in my opinion serve better for tracking headphones, and hours and hours of recording/tracking. ATH-M50 have an insane amount of low end, and bumped highs, so they are not accurate for mixing at all.
RP-21 have bigger holes in the ear pads for your ears, so in a sense they could be more comfortable if you have big ears, but overall comfort goes to ATH-M50, because the pads are softer.
RP-21 ear pads seem just about the same good quality, just slightly less wide.
RP-21 are lighter set of headphones, so it's a big plus for long session.
I'm very satisfied with the purchase. I'm going to be wearing RN-21 when tracking vocalists, players in the same room as I'm monitoring/recording, and these are the perfect choice.
If I had a choice of having 2 sets of headphones: 1 for tracking and 1 for mixing/checking the mix, my vote would go for
Equation Audio RP-21 and
Audio Technica ATH-M50 