You can experience this by the position of each one and some software update in a general way and as far as I know when you put a pair of Subs in your Home Theater each one reduces the power in -3dB to make the same volume with two as with one but now if you touch the settings if you increase the Sub volumes with the settings it will have more power with less increases :)

Also I experimented as you that the Sub Mini is more powerful that a single Sub4 but I reiterate, using and moving the settings the Sub4 will have more bass and this also occurs with the pair :)

​@leo07 - Yes, I do agree that raising the EQ settings helps some, but to an extent. I still feel, for some reason, the Dual Subs are getting turned way down in power somewhere in the software.

Here is what I have my EQ settings set at,

TruePlay: Enabled (Quick-TruePlay)

EQ
- Bass: + 5
- Treble: + 6
-Loudness: On

Surround Audio
-TV Level: + 6
-Music Level: + 6

Sub Audio
- Sub Level: +14

Height Audio
-Height Level: + 8

As you can see that I am almost maxed out on what I can raise the Sub Levels at. I wanted my Dual Subs to do all the heavy lifting, but because I am getting this nerfed effect, I have to turn the EQ Bass up to +5. On my Beam 2/Sub Mini setup, I could leave the EQ Bass at -1, and it would shake the room. 

Hopefully, there is an update coming soon that brings the Sub 4 to its fullest potential when paired with another Sub 4 as Duals. I have read about a good number of people with this issue, and I believe it's something in the software; it has to be for the fact that when I disconnected the pair and only tested them one at a time, they were performing at a much higher level.

To check everything is operating optimally you could generate a diagnostics report and contact Sonos Support? 
 

Is it possible the Subs are partially cancelling themselves out by facing each other? Try putting one in a different location in the room. 

I don’t think they are canceling each other out, though that is a good suggestion.

Here is the layout of my room to further explain where and how they are placed.

This placement is actually what many companies who sell Subwoofers recommend to get the most out of Dual subs.


I just got home, and again, I unplugged each sub just now and tested them individually. When one sub is unplugged and only a single sub is playing with the system, it is getting much more bass on each sub when tested individually. I would say it’s 50% more bass with a single sub at the same EQ levels and volume compared to when they are both connected.

I will follow up with Sonos and open a ticket to see if they can help troubleshoot this issue.



 

Try inverting the phase on one Sub, if things improve it was cancelation.

Then reset the sub you inverted and invert the other and see if it is better or worse, this this to do the best match to your other non-Sub Sonos low frequencies.

A single Sub is enough of a challenge to get set up for optimal sound, dual Subs can be much more challenging and less forgiving of less than optimal placement and settings.

Does that mean “Sub-optimal” or is that too much of a pun? 

Dual Subs always have the same phase setting and the phase control is disabled when Trueplay is enabled.

Honestly, having the subs on opposite walls and diagonally from each other is about the worst thing you can do.  You are creating null zones where the large bass waves cancel each other out, and there is no wonder why performance is suffering.  Try putting the subs right next to each other (or one on top of the other) and set the phase to the same on both.  Co-locating them will cause the waves to go out in the same position, and there won’t be as much cancellation.  

​@jgatie With TruePlay enabled the phase control button is not an adjustable option. Do you suggest I keep TruePlay off? 

​@jgatie , according to SVS and a couple of other Subwoofer/Home-Theater forums, the opposing diagonal setup for Dual Subs provides some of the best results for Sub Placement.

SOURCE: https://www.svsound.com/blogs/subwoofer-setup-and-tuning/75040195-why-go-dual?srsltid=AfmBOooQr8V2sdZrYCmx09snxoWVaNSfd2I53rNRFyR8cp7YlMwUhv6J

 

The first reply I received in this forum was that when the Dual Subs are paired, they lose -3dB. This is the most accurate way to explain what I am experiencing, but a lot more extreme. I am losing 40-50% of the power when they are paired as duals compared to when they are set up indvidually. When I set them up, I performed a sub crawl and placed them in the 2 spots that sounded the best. I also tried switching them in and out in the same diagonal, opposing placement, but each sub individually tested in each of these 2 spots. They both performed the same individually, so I know that one is not a dud. It’s just when they are paired as Duals that I am experiencing the loss in performance.

I did open up a ticket with support last night. The Support Tech walked me through all the troubleshooting steps, and I believe they connected to my system to check the performance, and from their end, all looked to be performing well when paired as duals. They did say that it is normal for one single sub to sound 40-50% more powerful than dual subs paired because the dual sub setup provides a more balanced experience. Maybe my expectations were just set too high when adding a 2nd sub. Perhaps I need to give myself time to adjust to the balanced sound instead of one sub booming from the corner. What's wild to me is that my EQ settings rattle the room at +6 Sub Level when it is a single sub, and to achieve the same results in the sub dual pairing, I need to set it to +14/15 Sub Level. It does sound good at +15. I worried that I would damage the subs at this high of a setting. The Support Tech ensured that they would not blow since the EQ settings allow for this much adjustable range.

I will keep my system at +15 for now as a Dual setup. Hopefully, one day Sonos allows for an update that allows them to perform at their full potential when paired. Even if it loses 3dB when paired, that shouldn’t be the case. For the best performance and customer experience, the full power should be available, paired as duals or not.

Believe what you wish.  But I would try the co-location first before I shrug off any advice. 

​@DonTheJuan please keep us posted having the exact same issue with 2 Subs and Diagonal Placement, based on sub crawl they are in the perfect spot

Try the co-location. It’s quick and easy. 

I notice that you have Bass and Treble elevated. This will require the SUB’s to be set higher in order to match.