I'm curious about this. Chris (WmAx) always preached stereo subs for music because he claimed that mono output that's been summed (in a receiver's crossover) from a source that is mixed for stereo causes cancellations within the sub. That mono subwoofer output is only acceptable when used with a source mixed for a separate subwoofer channel. I may be over simplifying this or maybe totally misunderstood him, but since I hold your opinion in high regard this seems like a perfect chance to pick your brain and learn something. Has anybody tested the hypothesis?
You would make me have to explain something complicated after a nice night out with friends at Paul's French restaurant!
First lets consider the recording process.
There are two schools of thought: - Intensity (coincident microphones) and phase difference (spaced) microphones. The more usual these days is phase difference stereo. I'm one of the few who still maintains that good intensity stereo with good speakers with minimal phase aberration gives the most realistic two channel experience.
Now the bass phase difference in a coincident technique is going to be minimal, but in the spaced phase difference technique it is going to be considerable.
Now lets look at how the LFE channel is derived.
THX specifications on the recording end state that the LFE channel must contain minimal information above 125 Hz. This makes sense because at that point you are getting to frequencies that are useful for localization.
The problem is how you get there. Because of puny satellites that people want to use because they can't control their wives, girl friends, other partners and the dreaded interior designers, frequencies to around 100 Hz need to be on the LFE channel.
So the THX encoder for movies takes the mono output buffered from stereo from the mixing console and applies a 48 db per octave slope, starting about 90 Hz to get the LFE channel down 24 db by 125 Hz. Now this steep filter causes horrid artifacts from the huge phase shift of the 48 db per octave low pass filter.
Now this system is not always used even in movies, and just about never for music productions with an LFE channel. The encoder low pass filter is switched off and a gentler low pass filter 24 db per octave at 60 Hz or 12 db per octave at 30 Hz is applied before the encoder.
Now it is strange, but few audio enthusiast or audiophiles ever give a thought to the recording process. They should, because it has a big impact on how things are set up.
Now the big point is that bass phase cancellations have already taken place in the creation of the LFE channel, because the channel is already down mixed to mono. The stereo channel and any other channels still contain all the phase differences of the mix for that channel.
Now when you set your receiver to output a bass signal to your sub, you have the same situation as an engineer who does not use the 48db low pass filter of the THX encoder.
So you still need to keep the LFE output down by 24 db at 125 Hz.
The receiver or pre pro creates the LFE output from a source with no dedicated LFE channel by sending the channel outputs to buffer amps and then combining them to mono. Such phase cancellations as will occur will happen at that stage. A low pass 24 db per octave filter is then applied. To have it 24 db down by 125 Hz means setting the crossover no higher than 60 Hz.
The phase shift of this low pass filter will be a delay of 180 degrees at crossover frequency. Any speakers set to small will be ahead 90 degrees at crossover frequency, as the receiver/pre pro high pass filters are 12 db per octave second order. The total phase shift is 270 degrees. That is the electrical component anyway. There are acoustic phase shifts especially related to sub ports and speaker ports. So it is a real witches brew.
The witches brew is much worse if we have subs in locations around the room with different phase from the recording microphones. The phase additions and cancellations will then take place in free space. There is evidence that allowing this to happen magnifies room peaks and nulls and is not a good thing.
There is consensus that spaced subs not part of an integrated speaker system, like my speakers for instance, should all be fed the same signal, and that means mono with no significant output above 125 Hz.
So the OP needs to feed his subs a mono signal summed from two buffer amps connected to his preamp outputs and then a 24 db per octave low pass filter applied no higher than 60 Hz. Since he will presumably not be using a high pass filter, his mains should have an F3 no higher than 35 Hz for best results. This is no different from using a pre/pro or receiver with the speakers set to large and a crossover frequency of 60 Hz. That is why WmAx view does not make sense.
Now if a speaker system is integrated like mine, then you can send a stereo signal to the bass division and I do when playing SACDs or when listening to two channel in pure direct mode. But when the LFE output is not used my low pass filter is 200 Hz first order transitioning to second order.
The above is just one of the many reasons I designed my system the way I did.
I realize that in this post you have made me confront some common misconceptions about adding subs to speakers. The biggest misconception is that using bookshelves with high F3 is made up for by a sub, and that larger more capable speakers with F3 down below 40 Hz are not necessary. I think you can see from the above that for optimal results that is not true.
The other myth is that the LFE signal from different recordings have the same characteristics. That is certainly not true.
