I think you will find that everything comes into play Derrik. There is no "one answer fits all" to your question on phase. There are mathematical equations to answer your question on phase but it would be way beyond my capability and would require allot of input. Distances will come into play. All HT rooms react differently so I'm afraid the answer lies with you trying it out and tweaking until you come up with your answer? You never know where that second sub could find itself?

How about stacked? I have seen it done.
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From what I understand the one PC+ would equal two PCI's in the lower freq? 20-30 freq range.> +5 +6 db over a single PCI.
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If it were me I would stay with the same like sub. One consideration possibly if your into HT big time is to sell the PCI (easy to sell) and go with the duel PC+ configuration.

if the subs will be within 5-6' of each other. It also might just be best to run one PC+.

Again >So much comes into play that only you will be able to answer.
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If you are just trying to fill a null, then the separate locations will be very effective. The phase setting on your sub can and will be a help but might--probably not be enough to cancel out all cancellations. Well I gave it a try.......GOOD LUCK.