Distributed Bass Array configuration

Why is there never any mention of level matching these DBA systems with the main speakers using an SPL meter?

There is. All over the place. I've never done it. I listen with ears, not meters. So why would I care what a meter wants?

Otherwise, wouldn’t it just be guessing what the volume level should be on the subs?

Is that really what you think? Do you "guess" what volume level you like? "I think I'll try 12 today. Wow that's way too loud! Guess I'll try 4?" Is that what you do? Of course not. You know perfectly well what volume you want. So why would subs be any different?

Otherwise, wouldn’t it just be guessing what the volume level should be on the subs?

For some Way Too Much isn't enough.

You're twisting it around MC, the whole purpose of level matching is to prevent you from trying to figure out what the volume on the subs should be. That's why its called level matching.

@mapman wrote:  "I think its a fallacy if some are asserting setting up multiple subs properly is easier than one."  

@golfnutz wrote:   "I agree, using the 'throwing darts' method isn't the way I'd install a $3200 subwoofer system." 

I realize that it seems counter-intuitive to think positioning four subs in a room is easier than positioning one, but consider this:  When you change the location of a single sub, you have changed the way ALL of the bass energy is introduced into the room.  With four subs, when you change the position of any one of them, you have changed the way ONE-FOURTH of the bass energy is introduced into the room.  By following a few guidelines you can probably end up with excellent results after one or two adjustments, if not from the get-go (which seems to be the case about half the time).  I include six different setup strategies in my guidelines, most of which are very forgiving, and all of which have been used successfully.   

Regarding setting the levels, either ears or measurements can give excellent results, but in either case (and regardless of the number of subs involved) finding the exact setting IS critical.  The reason is, the ear's heightened sensitivity to changes in SPL at low frequencies.  A little bit of change in gain on the subwoofer amp translates to a disproportionate change in PERCEIVED loudness.   So it may take a while to find that magic setting on the amp's gain knob.  Note that measurements alone may not be enough - Geddes and Toole and others have observed that most listeners prefer slightly elevated bass compared to the rest of the spectrum, but the amount of elevation preferred varies.  I got excellent results fairly quickly in a challenging situation by using measurements as a starting point and then having an experienced bass player guide me as I fine-tuned the settings.  

Note that the improved in-room smoothness of a distributed multisub system allows more flexibility in setting the level.  It is the peaks which stick out like sore thumbs and are the limiting factor in how loud we can set the bass before it starts to sound unnatural.  Smaller peaks means that we can set the level a bit higher for more satisfying bass because we don't have to sacrifice a truly satisfying level across the rest of the bass spectrum to avoid over-energizing the peaks.  

Duke

I have found the best and most efficient way to set up a sub is to measure first to get things in the ball park and then fine tune by ear from there.
What I get out of the whole DBA thing is measure before doing anything. It may be a waste if peaks are not a big issue. There will always be some though so it’s all a matter of judgement.
If the goal is to smooth things out at just one spot, DSP is probably the most straightforward solution. Or a good old fashioned analog parametric equalizer.
For smoothing out across the room then the more heavyweight dba solution might make sense, but only if the mains are capable of delivering across the room as well ie they have wide dispersion characteristics otherwise you might have smooth bass across the room but everything else falls short.

Then there is the alternative where you have multiple subs set up not  primarily to smooth out response but for best soundstage (smoother response is a secondary benefit).  That means if 1 sub front dead center,   2 subs at or near position of mains, 4 subs the two behind you are set to a lower level than front.