Get the 8 ohm cab. The difference in volume you will not notice. With 4 ohms if you need more volume you are stuck with one cab, and are more likely to blow the speakers.I will upgrade to gk cx410 4ohms
But with the 8 ohm you can add another 8 ohm cab later, with that you will notice a huge difference in volume.
With speaker size it depends on make and model not size. Even with two 15's buy the same company one model can be bright and not much lows, and the other
model has more lows and not so bright, or one that can do it all.
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