Yes, I really like the clarity of the RE-400. It's becoming increasingly apparent that the increased midrange clarity is what I was missing from all of the other IEMs that I've tried. Well, I guess my Brainwavz M2 and ProAlpha had midrange clarity, but they also were more strident/peaky than the RE-400 and with a much more congested soundstage (i.e., less separation).
If I will miss some clarity with the Fidue A63, do you think there is something that comes closer to the RE-400's midrange clarity that also contains the added warmth of the A63 for $75 or less? I understand that I'm not going to get the same amount of clarity as the RE-400's, but I'm wondering if there is something a bit closer to the RE-400 than the A63. Using your spectrum, if the A63 is like halfway between the GR99 and the RE400 in terms of midrange clarity, is there something that is between the A63 and the RE400? Someone suggested the Zero Audio Carbo Tenore to me, but I don't think you've heard that one yet (from reading back through this thread).
The sweating thing has never been an issue for me, so no worries on that front. I've been using non-sport IEMs at the gym for years and have never had a problem. Of course, now that I said that, I will probably develop a problem!
Seriously, though, I think it's smart to not use the RE-400 at the gym given the durability issues.