Apples against Oranges really.
The M6 is the better of the two. It is slightly v-shaped but the Bass is tighter and isn't a fart cannon like the XX's.
The Mids while slightly pulled back has WAY better detail, They treble is nice and crisp and reaches out pretty far, it can have some sizzle on some songs but can be fixed with the right tips. Another plus is the nice 3-D effect the M6 gives out.
The XX on the other hand is a Bassy mess, it is so smeared the Mids get totally washed out and the treble is very metallic sounding.
Don't believe the hype machine, their $20 for a reason..If you like a bassy IEM and are on a budget, save up about $40 and get the Soundmagic E10, The XX's aren't even in the same game...If you want another balanced set with Great build quality check out the CW31, It's $30 and better then all mentioned IMO..