Single vs. Dual Subs

What do two subs add to the music presentation that a single dialed in sub is missing provided that single sub is dialed in perfectly for the listening position?

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You answered your own question. If ONE sub was dialed in perfectly, the second sub will add ZERO.

The first one is dialed in PERFECT. STOP!! :-)

Second who said the first sub can’t have 6 active drivers and 12 passive radiators, BUT all in the same SINGLE 50 cubic foot box. :-)

One box fits all in some cases.. At one time I built 500 lb enclosures that were 22 cubic feet. You only needs two in a 50 x 100 x 12 room.
2 18" 2 15" and 4 passive 15" radiators.. 500 watts was plenty..
BTW they were Bins not subs, they were CUT on purpose at 28 hz and below.. 28-80 hz. The room still had 4 72" Narrow baffle bass columns 80-280hz. 6 8" MB drivers per enclosure

It’s about pressure in a room for BASS. 60-80hz and below not so much. WRONG kind of pressure.. High sub pressure can wear you out.. I don’t like it when I have to open my mouth to EQ the pressure fast enough to get clarity in the mids...

I've built single driver slot loaded bass bins.. ONE would fill a hole two car unfinished garage to 110+ db.. with 200 watts MAX. Boom Boom in the room room.. Modified Jensen Imperial Subs.. I've seen them look like an upright pianos, they were so big..

When you see SUBs working well into the BASS region, that’s when it get messy and bloated..

Regards

The technical theory behind multiple subs stacks up and there’s a lot of support for it. The only caveat I can see is that to make the most of it you need to be pretty flexible about where those subs go, if they just go right behind the L/R speakers then it may be an imperceptible improvement. Also it’s worth remembering that your room is a 3d space so different height placements should be considered as well. Basically symmetry is most pleasing to the eye but probably the worst way to lay out your two subs. If you have the freedom to put them wherever you want then there’s a good chance you’ll hear the benefits.
You’ll also want to do a bit of research into how to systematically set them up either by ear or using a mic.

The technical theory behind multiple subs stacks up and there’s a lot of support for it.

People I respect a lot like Toole propose this, but they often are talking about large rooms.

Conceptually this practice makes perfect sense to me. However in simulation, I just can’t get it to do much. So I’m unable to recommend it as much as I used to. If only I had the time energy and money to buy another 15" sub and shlepp it around my room. :)

I defer to those with more hands on experience than I.

Of course, you all know, the middle course here is bass traps plus EQ. The traps reduce the nulls and peaks, and the EQ takes care of what’s left.  Unlike multiple subs in a room, this is an approach which I have enough experience personally to recommend.  Of course, I'd never deny anyone the right to spend money, and I acknowledge my limitations in experience.

I agree with @mijostyn - my experience is that adding a second sub resulted in significantly better sound compared to using a single sub.  The designer of my speakers (Michael Kelly with Aerial) has commented about the benefit of multiple subs in the Aerial literature, in addition to saying more than two subs is even better.  As full disclosure, at the time I added the second sub I also switched from larger, (almost) full-range, bass reflex floor standing speakers to moderate sized, acoustic suspension, stand-mounted speakers.  The stand-mounted speakers are solid to about 40Hz and the subs roll in at that frequency.  The result is a vastly superior ability to tune the bass to the room, lessening of nodes, absence of boominess, more power/drive in the low frequencies, and greater clarity up through the midrange.  Both sets of main speakers utilize the same midrange and high frequency drivers so the only difference is in how they handle the bass.  It is a bonus that the subs have their own remote control, making it easy to dial the bass volume up or down to sound best with specific musical content.

For two channel stereo the best to the worse is: A pair of woofer towers, a pair of front firing subs, a pair of down firing subs, a single front facing sub, one down firing sub.