You are 100% right that a single bad unit says little about their quality control. A few pages back, some other people said that they were having some issues, and someone even suggested that there might be quality issues with the batch on market now, but only time will tell.
While your explanation of the sound card issue is plausible, it's odd that the issue occurs in both ears and that the phones work fine with my E7. On the other hand, you are right that it is possible that the wiring issue is affecting the overall sound signature, and that this accounts for my distaste.
On comparing the AD900 to the m100: well, if what you mean is that they sound completely different, I would agree. The sound signatures could hardly be more different. So you're right that 'they will never be similar.' On the other hand, I don't see what's wrong with comparing them in order to consider which sound I like better for home use and for the music I tend to listen to. (For example, m100 is vastly superior for most electronic music and rap. I was also impressed when I listened to Kid A. But for classical, solo piano, classic rock, female vocalists, reggae, jazz, etc., the AD900 wins hands-down. Neither of them are very good for rock and metal.) I'm failing to understand why this sort of comparison is off-limits. It is very useful for those of us who can't afford to own a 300$ portable unit and a 300$ home unit at the same time.