Mixing subs is going to give you mixed frequency responses just as he said. The problem is each sub is going to have different responses in different boxes. They will also have different transient responses and that can isolate each sub to your ear. Not a good idea. Having different subs crossed differently to play different frequencies is fine if using a steep slope. When different subs are combined it normally just sounds like mush to me. All kinda blends together a little. Each sub will distort differently. Each sub will have different power handling. Each sub will have different impedance spikes. Each sub will have different power compression. And as far as it being louder...I doubt it. Acoustic coupling wouldnt be as great. And with the different TR they could actually cancel each outer a bit if the phasing is off enough.
A jet plane engine at full throttle is pretty loud, but I sure as hell don't want to listen to it. In other words, having the "loudest" system doesn't always mean having the "best" system.
Depends on who you ask. It works the other way as well. Just because you have the most accurate sound stage with the lowest distortion and a completely flat frequency response doesnt mean its the best. There is no best. "Good sound" is subjective.
Consider this... Every sub has strengths and weaknesses. Inherently, if you try to implement one sub that has a strength in one area, and is weak in another... And try to also implement a sub that's strong in the areas that the first sub is weak in, but is weak in the areas that the other sub is strong in... Some people think this means you get "the best of both worlds". In reality, you only end up with "the worst of both worlds". The reason? When you bring the system up to a level where one sub is just about to really excel and show it's strength, the other sub is inherently reaching it's limits, and beginning to sound horrible, show signs of distress... And as a result, you can't turn it up farther, without it either sounding horrible, or even damaging the sub that's reaching it's limits. You'll never end up really experiencing the strong points of each sub... ...instead you'll just be perpetually constrained by the weaknesses of each. And that's also why you NEVER see this technique employed in competition systems, or recommended by professionals.
