I've recently purchased the RE-600 and I must say that i'm extremely impressed with them so far. I've been a long time user of the Etymotic ER-4P/S for many years and more recently the Westone W40.
To my ears, the RE-600 are able to match the treble and resolution of the ER4 but with a much wider soundstage and fuller/impactful bass, yet are much less prone to sibilance. Isolation still leaves a lot to be desired, but comparing against the ER4 probably isn’t a fair game lol.
The W40 has more impactful bass and great layering, but sounds confined. Treble is a tiny bit less forward vs the ER4 and RE-600. As though there was a fine veil over it. But honestly, it might just be me since I much prefer an analytical sound. Perhaps if I had never listened to the ER4 I might fall in love with the W40 lol
Later on, I got the HM-901 DAP and listened to the RE-600 in balanced output. I was blown away! The soundstage was opened up even more! In certain tracks, I felt as though I was an audience at the stage. It was very rare to experience such a soundstage on an IEM. Vocals weren’t so ‘in your face’ and sounded more smooth and laid-back. I don’t see how one would dislike this pair of IEMs. To fully realize the potential of the RE-600 I highly recommend running it in balanced mode!