Sot, the receiver used the data from the Audyssey measurements to set the speakers "Full Range" because apparently in your room there was good response as low as 40Hz. This doesn't mean that it's best to leave it that way, however. Setting a higher crossover manually(typically 80Hz)allows your fine sub to handle more of the frequencies it does best and take part of the load off your speakers so that they can play the higher frequencies a bit more cleanly. Also, as Randy pointed out, Audyssey XT has higher resolution filters in the sub channel than the speaker channels. As a result, the correction in the lowest bass frequencies can be more accurate when the sub rather than the speakers is handling them.
