Originally Posted by
ccotenj 
nope... it's not "my theory", it's pretty much basic acoustics...

consider this... my main speakers (which are towers) have
measured in my room response into the mid-30's... they are set as "small", and crossed over at 80hz...
also don't forget that a crossover is not a brick wall, it is a slope...
as to "why"...
the frequency range in a "normal home environment" can be divided into 2 parts... one where the wavelengths "fit" in your room, and one where they "don't fit" in your room...
anything above 200hz is in the radiant zone... we don't care about that right now...

other than to realize this is where your "imaging and soundstage" primarily come from, and ideally, your main speakers position is optimized for this...
depending on the size of your room, below 200hz, room modes start to effect what you hear... below 80-120hz, room modes dominate...
because of this, the positioning of your subwoofer can (and does) have ahuge effect on what you actually "get"... and due to the way room modes work, the "good spot" (or at least "least bad spot") is almost never where the main speakers are...
for example, you might corner load your subwoofer (as most do) in order to maximize output... or you might go with a pair to smooth output...
all the above also ignores the potential for cancellation/reinforcement/phase difficulties/power compression on the mains/etc. when running "plus"...
there's more (and what i gave you is a gross simplification) but hopefully that gives you a basic idea...