I think this needs some clarification:
Sibilance and 'T''s and 'P' issues happen at the source and are mainly to do with the artist being too close to the mic or not using the best material pop shield and similar
IEM's don't posess a fault that can reproduce this issue from their own side with music
Because these IEMS are very good quality, well tuned but have an over all 'bright' signature you will find that these type of faults may be revealed on the recording more than with other headphones.
Vinyl rips tend to suffer more from sibilance than other types of recording
Anyway - I LOVE my A83's - the bass blew me away as well when I first heard them
To add: Especially during the Mastering stage: the sss's are one the hardest issues to avoid (in general) and deal with when they have occured without overlly effecting the other aspects of the audio - Im kind of surprised that this issue seems to have escaped so many recording engineers during sessions?