Difference between a Mountain and an XLM is the weight, 350/400g less on the XLM (for the same frame's size).
The Tune cranks have been around since the mid 90's I think.
I have had conversations where the bod has suggested that Tune were involved with Topline and possibly Cooks in the design of their cranks - I have no idea if that's actually true.
I saw your Merlin - funky handlebar/barend arrangement??
Wanted: Trek XO2 54cm
Running:- Orange AluO; Orange Gringo; Orange Evo2; Trek Madone 6.9 SSL; Trek 1500 SLR
In Progress:- Orange Elite; Trek Elite 9.8
Selling:- Orange Sub3; Orange O2
Sold :- Merlin Titanium; Specialized 99 FSR Elite; Orange Mr XC; MountainGoat WhiskeyTownRacer; Orange AluO