Conventional opinion says to calibrate the sub to the same level as the speakers but I'm not convinced that's correct.

Our ears are about 40dB less sensitive to a 20Hz tone than to a 1KHz tone (that's ten thousand times less sensitive) for normal listening levels. Given our ears' decreased sensitivity to lows, we need to turn the bass up. We can't turn it up 40dB because as the frequency increases, our ear becomes more sensitive and the bass would be too hot. This is where a loudness control would be very useful.

I've found 6dB above my speaker levels to be a good compromise for my room. It sounds just right for music and I have earth-shaking LFE during movies.

As for the phase, the M80s are out of phase with the input signal by 45 degrees at frequencies below 50Hz as shown here. We'd have to know the phase response of the 600 to do some analysis. The phase is also dependent on the distance from the listener and also the relative orientation of the speakers to the sub. And of course like Jake said, the delay setting in the receiver.