Roti, too much won't be until the 1604 would grossly distort(the cable has nothing to do with it)on some extreme peak. Nobody can tell you when that will be. Bear in mind that at a comfortably loud listening level the M60s will be using about 1-2 watts from the 1604. On very brief extreme peaks 50 watts or more can be used, but your ears may give out first. It depends on the volume of your listening room, how far away you are, and how high the peaks are on the music that you're playing.

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Enjoy the music, not the equipment.