HT-21 is clearer than the PortaPros and is more of the sound signature you are looking for (powerful mids, no bass bloat) but in isolation there's just no contest between them and the M6. The M6 is a fully sealed IEM and with well-fitting tips will isolate a good amount more. A compromise might be the CX21 model from the clarity series which has more of an HT-21 sound in an in-ear package, or better yet a CC51.
Am I right in saying that the M6 will probably give a closer sound to the PortaPros in that it has strong bass and recessed mids? How is the bass on the CX21? I was listening to a friends sony mdrzx100 headphones, and the mids and highs were very good, but the bass seemed a bit lacking, as if some notes were missing. Will the CX21 have this problem as well?
One more question, will the CX21 give better detail and separation of the music but less soundstage imaging than the M6?
Edited by ICSX - 10/18/11 at 10:32am