Adrian, per Audyssey, you don't want to reduce the crossover settings that Audyssey finds, it is ok to increase a value. Davekpro's Audyssey set is side Q's to 90hz, and rears to 150hz, because the rears had no wall behind them and didn't get any reinforcement like the side surrounds which were wall mounted.
Per Audyssey:
"Raising the crossover frequency from the calibrated setting does not affect the channel correction implemented by Audyssey.
Lowering the crossover frequency from the calibrated setting is not recommended. Audyssey will not provide correction to the satellite speakers lower than the frequency it measures as the -3 dB point.
