If the intent for the proposed VP160 is to complement the M60 first and foremost, then I would think the combination of having the woofers to the outside and the tweeter and mid in the central position with the tweeter above the mid would likely be the best combo. As Tom implied, it will probably call for a slightly boxier cabinet with a wider baffle so I'm not sure if this would be desirable or not. The other alternative would be woofers to the outside with the tweet and mid mounted slightly diagonal to each other in the centre. Ideally, you might want a mid on each side of the tweeter but then it almost becomes a VP180 (with only 1 tweet difference). Several different combos to consider....

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