Eric, if you set your 1905 at a 80Hz crossover you should get the internal crossover on your sub out of the way so that it doesn't interfere with the 1905's crossover. If the sub has a switch to bypass its internal crossover, use that; if it doesn't, turn the sub crossover dial all the way up to 150Hz or whatever the highest setting is, to get it as far out of the way of 80Hz as possible.

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Enjoy the music, not the equipment.