I took the lid off, couldn't see anything suss, ao decided to wire it up in the car again. This time i took out the crossover and went straight from headunit --> amp. Worked beautifully. I have no idea what the problem was before - perhaps somehting funny is happening with the crossover that one amp doesn't care about and the other one does (+/- out of phase??)
I will tell you what happens when I connect up the crossover again, but for now i am happy my amp is not stuffed! thanks again