Just received these in the mail from Massdrop. I've been listening to the RE400's for about a month now, and have really grown to love them. But in comparison to the RE400, the RE-00's are definitely a great improvement. What stands out most to me with the 00's over the 400's is the sparkle in the high end that is lacking in the 400's. I am also more able to distinguish individual instruments through the 00's. A specific example was when listening to Haydn's The Creation. There is a section where two flutes alone play a minute or so in. In the 400's I can tell that there are two flutes because of the slight delay of one of the flutes coming in, outside of that I wouldn't know there was more than one instrument playing. I listened to this same section in the 00's, and I can actually hear two distinct flutes. I'm fairly new to the audiophile arena, so I was pretty floored that was even possible. That I'm getting for $34, what my $79 RE400 are not capable of is amazing to me!!! Definitely loving these so far. The bass response is not far from the 400's. Possibly just a smidgen less responsive at the low end. But again there is a detail at the low end that in the 400's seems a bit mushy in comparison. Female vocals have a beautiful "shimmer" to them, also not heard in my 400's. These also sound less in my head, than the 400's. I don't hear left ear and right ear, but feel more like the music is in front and around me. Awesome pair of IEM's. Would love to know if there is anything comparable to these for the value.