Dear Espen,
You are wrong in your assumptions. It is actually quite easy to predict how a single cornerplaced subwoofer will behave in an acousticly untreated room in terms of room resonances and standing waves. This has very little to do with the subwoofer itself and its cabinet stiffness. This is in relation to how the room behaves. I also don't agree that there are countless ways to integrate subwoofers with fullrange speakers. I actually think there is only one theoretically correct way and that is to let the coherency of the main speakers shine through and let them play across its entire bandwith without much overlap to the subwoofer and extend the bandwidth of the complete system by crossing the subwoofers in with very little overlap.
Like the knowledgable people on this forum has tried to tell you and the DIY guys on HFS will never understand it is very different to create a big tower multidriver DIY subwoofer and integrate that with a main speaker not playing fullrange (like 8x12_TOM has done for instance) and to integrate a full range speaker with a subwoofer containing two drivers.
Please don't misunderstand me. I am sure your solution sounds wonderful to you and is the right way to do it for you to acheive what you like. You have to do what you like best for yourself, but the way you and have solved it (influenced by DIY guys not knowing what they are doing I have no doubt) is not the way most successfull integration between fullrange speakers and subwoofers are done. Take a look at what David Wilson has done in his home and read the review of the Audio Physic Rhea by Michael Fremer in Stereophile. My post was a comment to your remark that you crossed over at a low frequency and would not benefit from one more QSub-15. Since you do not cross over at what is considered a low frequency at around 30Hz it is wrong to assume you would not benefit from one additional QSub-15. The reason being that you play the QSub-15 across such a wide frequency area that even though you might not hear the location of the subwoofer directly (with your crossover point I do however think that is highly unlikely), I am quite sure your room will resonate or have standing waves created by the subwoofer which will identify its location. With a lower frequency crossover point and the subwoofer playing across a narrower frequency band the chances for this would have been much smaller. By adding a second subwoofer you would both have been able to cancel out some room artifacts created by the subwoofer you use now and you would have been able to distribute the artifacts the subwoofer is creating across the room and making it much harder to identify its position.
Another big advantage with two subwoofers you are missing out on is that you have a stereo system but you play the lowest frequencies summed up through one transducer. With two subwoofers the deepest bass would also have been played stereo from two transducers like the main speakers.
BTW, You critisize me for telling you my opinion without having heard your room or your system, but isn't the real truth that you are trying to predict how it sounds with two QSubs without ever having tried two subwoofers in your system. I am willing to bet that two QSub-15 will sound much better in your room than the single subwoofer you have now. It has nothing to do with increased output level. You might in fact have to turn the output level down after you have added the second sub.
