Mysterious Slap Back On Drums When Playing Through My Pedal Board (Synths Work Fine!)

So confusing. All of my pedals are off, yet still when I play drums through my board there is a slap back delay on each drum, no matter the patch. This only effects drums and the synth engines work fine. When I play drums patches over the built in speaker, or through headphones it is fine. Any thoughts?

I would say something is funky with the board as the obvious answer, but the fact that the synth engines operate normally is really throwing me for a loop.