Cannondale Super X or Crux for Colorado Gravel Events vs. True Gravel Bike

Hey Gravel Grinders - my first post here. I fell in love with gravel last season just riding my aluminum, rim-brake Specialized Crux (CX bike). Now I want something more substantial and higher performing (stiffer, lighter, faster) for next season. I also realized I need disc brakes for long, hairy descents (mini-V brakes just don't cut it). I'm planning on several Colorado gravel "races" and events like Roll Massif Wild Horse, Crooked Gravel, FoCo Grand Fondo and SBT. 

I thought I needed something like a Niner RLT RDO (currently at top of my list as well as the Norco Search XR). Although I like the idea of being able to throw some 650b wheels with 45c tires in there on occasion, I really see myself riding lots of road and finer gravel 95% of the time. I'm also drawn to the SuperX or carbon Crux because they seem to run lighter builds, it bewilders me how many of these gravel set-ups are North of 19 and even 20 lbs.!!

Does anyone know if you can actually put 650b's with 45's on a SuperX or Crux? Are there merits to a true gravel above and beyond a CX bike for my purposes? The SuperX has a 71 degree HT angle! Pretty damn slack. Shouldn't that bike handle gravel races better than even some "gravel" bikes?  I appreciate your thoughts and comments!

Before I say anything else, let me state there is nothing wrong with using a Cross bike for gravel.  People have and continue to use them and have won races and enjoyed them very much.

A gravel bike will generally have more stable comfortable geometry for long days in the saddle on gravel.  They have lower bottom brackets, taller head tubes, slacker angles, usually longer chainstays, shorter seat tubes for longer seatpost extension and more tire clearance.  The trade off is that the bike feels slower handling and not as racy so it might not be ideal for someone looking for a racy “feeling” bike.  You have to decide for yourself what you want in a bike.  But there are bikes like the 3T Exploro and Cervelo Aspero that are much more speed oriented.  

As far as weight, built with the same parts a gravel bike is going to be the same weight as a disc cross bike, cantis obviously result in a lighter bike but few people want cantis on a gravel bike.  Gravel bikes usually come out heavier because you have wider rims and tires(usually with heavy puncture protection) that weigh more than the comparatively narrow rims and tires on a cross bike.

i doubt you will find any pure Cross bike that will fit 45c or larger 650b tires.  There are some bikes made for Cross and Gravel (Ibis Hakka, Santa Cruz Stigmata) that I think do.

Crux was at the top of my list for a gravel bike, and I ended up getting a Canyon Inflite (16-17lbs depending on wheels/tires).  I love the goat like climbing, agile handling, and the high bottom bracket.  However, I would not enjoy this bike on hairy descents or anything where I might come close to drifting.  Hard 20 mph turns on loose stuff requires some attention.

I put 50mm (sometimes 54mm) tires on the front of my bike.  I'm not quite losing sleep over the 650b thing when I can go so fat in the front (also slackens the head angle and adds about 10mm trail).  Food for thought.

A lot of cross type bikes tend to have parallel chainstays that flair out late - meaning with a smaller wheel you don't get any extra clearance.  

Have you considered the GT Grade?  The most recent model will handle 650b wheels and you get some nice rear wheel travel due to the flex of the decoupled seat stays.  I have the older version which is still really nice, but I will probably upgrade to the newer one soon.  https://www.gtbicycles.com/usa_en/bikes/road/all-new-grade

I loved the grades I test road, but the tire clearance was too small.  Nice that they have fixed that (that bike was a ahead of its time).   Can't say I liked the aluminum build, but the carbon build was sweet.