this sounds promising, finally some good news!
although, RE-400 was said to be a touch sibilant here - is this gone on the RE-600?
any comparisons to something like Phonak PFE series, or perhaps ATH AT-CK10/CK100? or UM2x RC? those are the only ones I currently have in my recent memory for IEM's. I am hoping they would be similar to RE-262/272 in terms of that.
I found the CK10/CK100 to have a little problem with treble, CK10 more sibilant than CK100, both a bit much treble but still quite nice. and the Phonak PFE had a few problems with sibilance as well.
UM2x I thought was quite nice, used the Comply tips for those. olives for the rest.