Can I combine cabinets with different speaker configurations and different Ohm ratings?
Yes, as long as you don’t go below the recommended ohm load per speaker output for the PowerStage model that you have. For instance, connecting one 8 ohm cabinet to one output, you could connect an additional 8 ohm load for the 4 ohm minimum. If you connect the 2nd cabinet at 16 ohms, the total load would be about 5.3 ohms, which is above the 4 ohm minimum. Here is a handy online calculator for figuring out the Parallel loads.