You gotta hear the pair of 1x12s to believe it ... I doubt you'd need the 15, these things have tons of bass. I left my head completely flat all night. If I'd boosted the lows ... there might have been trouble!
With the 750, you could also do 3 1x12s @2.6 ohms
With the 750, you could also do 3 1x12s @2.6 ohms

