User blog comment:B214/Volume 21 info & spoilers/@comment-26821288-20160321152502/@comment-26285797-20160321162047

Considering the BxB is Balance Breaker, the full unleashed state of Sacred Gear, CxC is power that exceeds the JD, and DxD is an evolution of the CxC, I believe that  AxA is an evolution of Issei's own body. Perhaps he has already awakened AxA, when his body was recreated from Great Red's flesh and Ophis's power. ExE maybe an artificial evolution, considering that they are mechanical beings. Perhaps ExE is the fusion of the organic DxD with an inorganic form.