Punit, I'll weigh in with my $ 0.02. The XC has the best midrange that I've heard (I'm not including my stat HPs). The 900 is more the fun can with deep bass and lovely highs which are not sibilant but the midrange is comparatively lacking. IMO, the XCs are more natural sounding.
These are my thoughts too. The XC can make your arm hairs curl with acoustic stuff like Avett Bros and Punch Bros.
To my ears the HE-6 are in the middle.
If I was to keep only one and have a proper amp it might be the HE-6 ,but if I could choose 2 it would be the TH-900 and XC.
I may try someday a speaker amp but I can't imagine them being better than the Master-9.