My friend just analyzed a sample of purported 6-APB and found it to contain 6-APB, caffeine, and a-PVP. This falls perfectly in line with the "Mad CAT Prolintane" a-PVP and green dye admixture which has begun to circulate. My guess would be that in preliminary tests 6-APB was found to be too serotonergic to induce truly fiendish meph type binging, so they added a DARI to increase the fiendishness quotient and, of course, increase profits. This would also make sense as 6-APB is a step toward IAP which, speaking in terms of fiendishness, is a step in the wrong direction.
EDIT: An even more obvious reason to cut the 6-APB with a-PVP is that a-PVP is much cheeper and less less difficult to synthesize.