I’ve got two modified Cracks and a modified C2A. All have approximately the same mods. I have two pair of the Hd650, original and the HD6xx.
So I’ve got some listening experience with all of those.
The C2A is better. Not by leaps and bounds but cleaner and maybe a bit clearer with just a bit of freq extension on both ends.
There are still days I like to throw the cans on a crack - there is magic there!
If you can swing it go C2A, you’ll enjoy the ride!