[UPDATE] This was written in 2014. Power supplies have come a long way since then, but back then, sourcing high current negative tip 9V power supplies was a challenge.

I wish we didn't have to go with the positive tip adapter. Unfortunately, it seems once you go above 200mA the standard seems to switch from negative tip to positive. We couldn't find a good supply of negative tip wall wart power supplies that would supply the required current.