I like how you summed up the carbon explanation, clever
My frame of choice coincidently is a Pivot Les but I can't swallow the $2000 entry for the frame. I've ridden the LES and really like the handling. Some other bikes that I really liked outside the realm of standard 70+ head angles with 17.5+ chain-stay lengths have been the Dback Mason and the Scott Scale in terms of hardtails. I would say 80% of my riding however is/has been on a single speed. This sounds weird but I've found the single speed is easier to ride then all my full suspension bikes, Scalpel Ultimate and Trek Superfly (Both now sold). Even funnier is that I ride to maximize momentum because I'm lazy. I don't want to pedal if I don't have to and try my best to carry as much speed into climbs so I don't have to stand and grind. Also, I find it easier to stand and mash then sit and spin.
Getting back to the geo numbers, for my riding style its easier to push the rear end out with a-little body English having a shorter rear end. The slacker head angle affords a more upright sitting position which for me is ideal rather then partially weighting the front end with steeper angles.
I don't have any experience on 26inch bikes and never had a roadbike prior to just recently to offer comparisons. Im still considering the frame despite not having the ability to run an angle reducing headset. Hoping that 120 fork with 55mm offset will provide the front end angles for an acceptable upright seating position. Nice that the frame is under $500 shipped.
Not too sure how I feel about these new "slack" XC bikes...not sure if its just more marketing to sell more bikes. My current ride has a 71.5 HTA, and to me it feels quite stable. The steering is fairly neutral. My Pivot has a ~68 HTA...and I don't really like it...at least not for responsiveness...good for going straight. The steering feels a bit sluggish and front end feels more flip floppy.
If you're trying to build a AM hardtail...I'd go with a frame that is designed for a long travel fork and a ~66 HTA.
This frame is just about a carbon (see what I did there?) copy of the 2015 Stumpjumper HT. The Stumpy comes with a 71 HTA and a 90mm fork. If you run a 120mm fork...you should be able to get the HTA to about 69 degrees. Mind that when you increase the travel...you're also increasing the bottom bracket height.
