Eh hem!...Subwoofers... What do ya know?

I have had a powered sub woofer in my audio system for 25 years and wouldn't have it any other way. Started out back in the day with Paradigm 20 monitors with Servo 15 sub woofer because of room constraints. Moved to new house and upgraded to Dunlavy SC lll's with the Servo 15 doing double duty 5.1 and two channel. When dialed in correctly it sounded amazing. No one, but no one back then that I new had two subs. Now in my retirement home with audio room contraints again I now have two separate systems in my small man cave, a dedicated two channel setup with a tube pre amp and tube amp and a 5.1 AV pre amp with a 5 channel power amp. For both systems I have the SVS ultra 13 Subwoofer. I don't have room for two unfortunately. But maybe I have something close to a sub swarm setup as my main speakers have built in powered sub woofers that will go down to 20hz. So along with my SVS Ultra 13 sub woofer the full audio range is pretty prodigious and well integrated with the help of Dirac EQ. I'm just amazed how well my system produces full range music, and for movies also, for that matter!

gochurchgo-

I guess I’d have to see what “placed ideally” equates to. Some light reading have the impression that one on each wall was the answer. How true that is I do not know.

if 2 $500 subs are better overall than 2 $200 subs then so be it. I’m just genuinely curious  as the “4 is better than 1” motto never mentions “good ones“ versus “ones that should work ok”.

Highly recommend searching and reading just about everything posted by Audiokinesis. Awesomely succinct, won't take long, just don't read fast there's a lot to absorb.

The quality/quantity thing is admittedly a hard one and doesn't sort out nice and clean. There's overlap, where one kilowatt 18" REL might be better than four 8 inch Hsu tubes. But not if total cost is the same for both.

Better quality is always gonna sound better. When I told Duke which 10" Morel drivers I bought he had no problem saying mine will kill his. By and large though my sense of it is for the vast majority of choices out there four of just about any of them will beat any one you can buy for the same amount of money.

Put another way the typical Swarm type DBA runs around $2-4k and I just don't think there's any one or even two subs anywhere near that much that can come even close. Just look at the comments. Pretty much everyone who's done this says its the best bass they ever heard. Not the best for $3k. The best, period.

Duke even had a customer choose his Swarm, and he had I think a $20k budget. Not for the system, just for the sub. Coulda bought a $20k sub. Listened to subs in that price range. Thought the Swarm was better.

That's why this is such an uphill battle. As if the physics, psycho-acoustics, mono, and "integrating" aren't hard enough, you got to try and convince people the concept is so powerful it overrides the need for great big expensive drivers. Oh, and lamp cord will do just fine. Its just too much to swallow. Even though its all true. 

I have Harbeth C7 mains, 2 Lyndorf BW2 subs, one in each corner, a Lyndorf 2170 amp. The Lyndorf can set a crossover for the subs/mains, I chose 100Hz 2nd order, you tell it the distance from the listening position to the mains and the subs so it corrects for time alignment, you then run the RoomPerfect room correction function and it perfectly integrates the outputs to match the room. So neat, tidy, easy. How does it sound? Bloody marvellous.
I have detected a few tracks that definitely have stereo bass, and that is a new experience to hear!

It would be helpful if you could describe your goals (music, video, both?)
better, best, best indeed!
room size/setup/placement options?
existing mains? new mains intended?

generally, not to make it work for you, just clues for us

I'm 71, have tried/learned several bass setups over the years.

I encourage tubes, and vinyl, reel to reel, so efficient speakers to reduce tube power needs. Entire system: darn good, not perfection, more money saved for more content to enjoy.

I go for darn good directional stereo bass for main listening,

listen to this, you will want directional bass. 

https://www.amazon.com/Double-Bass-Niels-Henning-rsted-Pedersen/dp/B00000G62F/ref=asc_df_B00000G62F/?tag=hyprod-20&linkCode=df0&hvadid=312683822382&hvpos=1o1&hvnetw=g&hvrand=18279210393491330306&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003512&hvtargid=pla-540093611693&psc=1&tag=&ref=&adgrpid=65958438881&hvpone=&hvptwo=&hvadid=312683822382&hvpos=1o1&hvnetw=g&hvrand=18279210393491330306&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003512&hvtargid=pla-540093611693
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Amount of bass: avoid too much, seems cool at first, but detracts from your delightful mains imaging, and loses bass direction. Extend system as desired, but not too much, yet definitely aware when you turn them off.
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Very careful consideration of room/placement/control options in advance
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music system, medium sized room: pair of subs and delightful mains:

pair of self powered directional subs. When mains not trying for low bass, mains are smaller, easier to place, and, importantly, main amp can be lower power, easier to use/try tubes.

very strong magnetic control of sub's woofers, avoid unwanted delay/distortion, preserves direction. Perhaps servo control of sub needed, depending on sub size/design.

solve location of mains without subs, then find nearby directional location of subs, refine sub crossover and volume control. 
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Sub remote volume control. Not common, or critical, BUT, remote control of sub volume is handy, especially home theater. Consider ease of use: internal sub amps with that feature, in-line volume control, home theater receiver easy menu access to adjust, exterior dedicated amp with remote volume. Individual remote volume control of pair of subs can make a difference.
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home theater, office (not big ones).

Single self powered sub, non-directional, added to smaller mains in home theater, or bookshelf in office. Same reason, mains can be smaller, easier placement.
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other considerations.

delightful mains, tweeter ear height, mid close to that, no ports.
ability to locate/move mains away from rear and side walls (less critical when they make less bass)

If need to move speakers back, forward only when listening, no spikes, heavy, i.e. sand/shot filled movable base, i.e. felt slide-able bottom.
 
16 ohm comments. my mains are 16 ohm, incredibly efficient. no matter what the math says about reducing the amps power, they are the loudest speakers I have ever owned, need no moe than 30 wpc (perhaps less, never tried). I think the other way: 8 and 4 ohm speakers increase the
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NEED for more power. Considering amp cost, size, weight, heat, large amps are generally to be avoided IMO. Especially if using or will try tube amp.

You won't avoid 8 ohms today, you can avoid 4 ohm speakers, and certainly, to use tubes, avoid inefficient speakers. Let the subs have their own power, remove bass needs from amp before it goes to mains, otherwise the mains will get the bass signal, they and amp will try and fail.