Fred, what you describe wouldn't necessarily indicate that the 400 has more output. It could be simply that the level control on the 400 "opens up" more quickly than the one on the 350. The result would be that the voltage output from the receiver that was let in to the sub amp would be equal or more at 1/4 to what the 350 level control let in at 1/2.

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Enjoy the music, not the equipment.