Got a chance to test it on his S9, Quality is about similar to my ear, but with different sound signatures. Cx-55 tends to be more high centric, being more sparkly and alot more in your face than the m2, but can also get alot more sibilant than the m2s. m2s are stronger in the mid to low range, especially the latter. M2s have Slightly tighter bass to me. M2s have a generally more warm and sorta spacious sounding sound compared to the Cx-55, which sound colder and more direct by comparison. I tested various genres in mp3, flac, and ogg format on the Cowan s9 in comparing these. In general I liked the M2s better for Rock, Rap, R&B, raggae, and music with similar styles to these, while the Cx-55 sounded better in classical, opera, etc..., stuff that don't have alot of bassy stuff and where the highs are more important. Jazz was a mixed bag, some styles sounded better on the M2 while others on the cx-55s. I did notice though that the cx-55s are more fatigueing than the m2s over extended listening on higher volume levels than the m2s, which makes sense since its usually the High frequencies that hurt your ears.
I still like both for different applications, but the M2s win overall as my Main pair for its less fatiguing and non sibilant qualities, especially since I listen over long periods of time while at work.
As for having to "see how the M2's hold against the CX-55s" do you mean that you haven't had a chance to compare them back to back? Or have you not received the M2's and are basing your comments on observation and reviews?
oh I have both, I just havn't compared them side by side yet. having too much fun with my shiney new m2s atm that I havn't picked up the cx-55s since. The M2s being more confortable in the ear with the foam tips probably has alot to do with it also.
I also need to steal ....er i mean borrow... my dads Cowan I-Audio X5 to do a proper comparison, since the cx-55s sound best on that beast.