The sound you are getting inserting the M6 is driver flex. It's associated with a lot of headphones, good and bad; it has nothing to do with price. As for sound, the sound isn't the best, but it's not the worst. The A151 is actually quite balanced. You should give the A161 a try ;)
I didn't know that driver flex can be found in a lot of headphones, thanks for the info.
I just bought the A151 about a month ago, but I'll keep in mind :)