Originally Posted by Kimwyn
chucky, you have it so right. Not only am I a novice at using REW, I am also overwhelmed by undertaking the project of trying to integrate a minidsp into my system as well. I remember it so well, using REW would take me hours a day. moving around the subs, recording measurements, then moving the subs again. It was all a lot of work, which at the time I enjoyed (I was around 25-26 yrs old) but now I simply don't have the time for (34 yrs old with wife and kids). So I honestly would just like to buy a sub (or a pair), drop them in a spot and run XT32. I could possibly use REW again, but it most definitely isn't anything I would be looking forward to doing.
I'm going to take a little different approach to your question. I think that out-of-phase issues can occur even with the same type of subs, based on room placement. When you mix subs with a differently shaped frequency response, due to both ported/sealed differences, and due to overall output capabilities, you may actually get some peaks/cancellations at frequencies where you wouldn't have had them otherwise.
Simply placing the same model subs in various locations in a room can have unpredictable results. Placing extremely dissimilar subs in a room just increases the unpredictability. That's why people are recommending measuring and being able to add some outboard DSP, if necessary.
My own perspective on this is that a single Cap 2400 will be so much more powerful than the dual HSU subs, that you may not notice them much. And, even if you stack them, their SPL will drop like a rock under 40Hz, compared to the 2400. XT-32 will not be your friend, in this case, because if you calibrate with all three subs in your system, Audyssey will stop EQing where the combined SPL of the subs drops by 3db. That means that you won't have any EQ below about 40Hz. And, you may really want to EQ the 2400 down to the bottom of its frequency response.
If I didn't want to invest a lot of time in this exercise (and perhaps even if I did) here is what I would do. First, I would set-up just the Cap 2400 and run an Audyssey calibration. Then, I would add a sub boost, play with the Low Frequency adjustment, and the other normal settings, and get used to the completely different magnitude of bass and tactile sensations that the Cap 2400 provides. I doubt that you can visualize the difference simply by reading other posts.
Once you have lived with the Cap 2400 for a few days, and have gotten used to it, if you are still curious, you can try adding in the dual HSU's to see if they add anything at all. I think it is likely that you won't even notice that they are playing, unless they cause some audible problems. There is that much difference in the Cap 2400's output at low frequencies. And, that is why I recommend that you get used to the Cap 2400 first, so that you will notice if they add something, or subtract something to your overall sound quality or bass experience.
When you do add them, don't run Audyssey again for the reason I mentioned earlier. You still want XT-32 to EQ your low frequencies. Just use Y-connectors (you will need two) to add them to the same sub out that your Cap 2400 is using. If you do it that way, Audyssey won't even know you have added them, whereas if you add them with the other sub out, Audyssey will invalidate your calibration and you will have to start over.
You can certainly approach this in a different way if you want to, but I think that this would be the best and easiest way to do this. It is the method that most people have found most effective when adding a ported mid-bass module to a full-range system. And, that is essentially what you are trying to do here, only with sealed subs instead.
I think that if you get used to the sound and feel of the Cap 2400 first, you will be in a much better position to decide by ear and by tactile sensation whether the dual HSU's still add anything positive to your audio system or not. If they do, you are ahead of the game. If they don't, you can decide where to go from there--either selling them, or investing more time (and a little expense) in measuring your frequency response and then in properly integrating them.
I hope this helps!