Both heads sound great. Are you clipping the M6?
While not hugely "louder," I think you would find the M9 noticeably "louder," but in theory, if you want twice the loudness, (10 dB more potential loudness at a given freq and distance) you are going need 10 times the power. 3 dB (still a very noticeable increase) more requires doubling the watts, so an M9 is theoretically less than 3 dB more. Make any sense?
However, the amount of gain the preamp provides might be different, which also could make a difference. BTW, the compressor and GEQ are quite good.
If you really need a lot more, it may be power amp time.
