Not unless the room is perfectly symetrical in every aspect including furniture, window treatments, etc. Really, I challenge anyone to honestly hear a difference when mixing two decent powered subs in the real world. Though I would agree that it is easy to hear differences between 2 dissimilar regular speakers , and even two of the same model speakers. As for phase, thats easy to test by flipping the phase switch on one of the subs, or reversing the input wires on one if no phase switch. Just make sure you are using a test CD phase test track first for a control and the subs are in phase with your other speakers. May be able to hear the differences if any between the subs with the main speakers off. Use a sound pressure meter to balance the subs volume from your sweet spot.