Positive Ground pedals (a common example being germanium boosts) use PNP transistors. These transistors tie your signal ground to the positive rail of the pedal's power supply. Most modern pedals use NPN transistors, tying your signal ground to the negative rail of the power supply. If you were to power these different types of pedals together you would short circuit your power supply. The Diago Powerstation and Micropower9 offer short-circuit protection, so this would cause the power supplies to simply shut off. On other power supplies this would likely cause irreversible damage.
