Dave, you're now using the correct settings, so nothing further needs to be done(it would be a good idea to carefully do the wiring connections over, anyway). Apparently there was a problem in that particular scene with the M80s running full-range("large")at the volume you were using, so don't do it.
As has been repeatedly pointed out, the lower impedance setting simply reduces the voltage(and therefore the current and power)which the amplifier can output, but UL(Underwriters Laboratory)fire safety requirements result in the suggestion in the manual. Despite that, for best performance the lower setting should never be used, as it cuts the maximum power available roughly in half.
Your 4311 should have plenty of capacity at high but safe listening levels, and there's no good reason to be spending money on more.
