Well if it's bright with the IE80, then it must be bright badly. :) I had IE80 and that iem's are pretty warm and laidback to my ears. The brightness may also have something to do with your amp. I like neutral amp and iem's, that way I know better the sound quality from the dap, which is considered by myself the most important piece of the puzzle. Of course, if the recording itself is bad there is nothing much you can do about it regardless of the gear. That's my opinion.
I just opened it up and re-done the mod with a fresh set of wires (pure silver 30AWG), it sounds much better now. There must have been some kind of short or bad connection before
I see that you highly recommend Kemet film caps, I may look into opening up the Fiio E12 at some point and replacing the WIMA ones that it comes with.
Has anyone opened up an E12?