Getting them in phase and getting the smoothest response is really the same thing.
Dr. Geddes already mentioned that he feels measurements are paramount to doing this right. If you can do it by ear, then great, I can't. Measurements are easy to do and immensely helpful.
THX reference levels, as I mentioned earlier, is 30hz to 80hz, right? at greater than 115db's. With that in mind, Dr. Geddes recommended subs would achieve that. Having said that, he started his comments in this thread by saying, he doesn't care what you use for subwoofers, as long as you have at least 3 and with varied Q's.
My take from our conversations and his posts is that this could be achieved in any number of ways. If you bought three identical subs (For instance), that were all ported like SVS does, where you can vary the tuning by plugging ports, just plug different numbers of ports, and you have achieved this. If you have one sealed sub, one ported sub, and a different sized sealed sub, you would achieve this goal. Even three identical subs spread around the room will be better than one.