Have you tried placing the Sub in a different location? Placement can make a big difference to bass response. 

Try changing the phase of the sub. I use the Sub-Mini in a 16’x20’ room with cathedral ceiling and it’s easily able to overpower the room and rest of the system. The subs are definitely not underpowered.

I have tried moving it a bit. But it seems the the sweet spots are always around the couch but never in it. 
I have a friend that have 1 10inc and 1 12inc. 
that is bass you feel. 
that’s what I am lacking, bass that I feel. I as bass in the couch ofc 

Good point. 
tho I have tried it too with the phase. A bit disappointed that you only can switch it 180 degrees. And not like 90 or 240. 
but I can say it sounds good just not a bass you can feel. 
would be awesome to se a Sonos sub to move more air.

Do you have the Sub or Sub-Mini? While I have the Mini, I’ve heard the Sub has serious power!

Switching the phase of the Sub changes the in-out movement of the driver(s) to out-in (i.e. 180 degrees), thus fully swapping the peaks and troughs of the sound waves produced.  If you swapped the peaks and troughs by 90 or 240 degrees, they would then cancel out and you would have no sound at all. 

As to the system not having enough bass, I have a Sub Gen 3 with an Arc Ultra and Ones for surrounds.  My space is a 30x15’ open floorplan with two hallways open to it.  I’ve never had it over 1/3 volume, and I don’t have the best ears in the world.  If you aren’t feeling enough bass, you either have an extremely large space, or you are looking for nightclub type output and will be shopping for hearing aids by the time you are out of your 20’s. 

Also, the Sonos Sub is pretty level in its frequency response curve.  It doesn’t have the “bass bloat” hump at around 60 Hz that cheaper subs do.  A level response is more accurate, but if one is used to the bloat, it may come off as lacking.  It’s not.  When it kicks in, you can feel it, but it’s not kicking in until the source calls for it.  

Presumably you have tried putting the Sub level right up, and the Bass level right up on the EQ?

If that’s still not enough bass then you must have the strongest molecular makeup of any human, and I would not enjoy living next to you.

I have Arc ultra, Sub 3 and 2 Era 300.

I ment isn’t it a potentiometer like others.
I am not searching for the hard cheap bass heavy sub. More like low deep “umph” a bit that u can feel. 
no doubt the Sub is powerful but not enough for me. 
Is it worth buying one more sub? 
 

I am how ever used to a car sub with 1200W rms…

and that you can feel, under 15hz 

You can add a second Sub (Gen 3 or Gen 4 is a requirement, I believe) in a home theatre setup. Whether it’ll match an in-car, small, enclosed space with a 1200watt sub, compared to an average-sized room with two Sonos Subs is another matter. I suspect you’ll be disappointed. 

Play a thumpy bass track and walk the room, close to the walls, with  handfull of Post-Its. As you discover a bass “peak”, mark it with a Post-It. Continue around the room and you'll discover a few areas with much larger peaks. Mark these points with “!!”. Stepping back you'll notice that that the Post-Its are more or less regularly distributed around the room. Midway between peaks there are “nulls" where bass is minimal. If you place a speaker or listener at peaks, you'll enhance the bass.

In my college apartment I put a chair on a monster peak. Bass seemed to explode from inside your body. This was the favorite chair for males. If I moved the chair a few inches, the peak was gone.

After some practice discovering these large peaks and nulls, you'll be able to discover smaller peaks in three dimensions throughout the room.

It sounds like you are looking for hearing damage levels of output.  Take it from someone who spent a good part of his youth standing 10 feet from giant Marshall stacks at hundreds of punk shows; hearing loss just isn’t worth it.  

15 Hz is out of the response range of most subs.

Also there is very little source material that has content at that frequency.

I built a fancy setup that included a dbx subharmonic synthesizer feeding Klispch LA Scalla speakers, through a kilowatt amp, that setup was pretty good down to 20 Hz. The dbx used a dsp to analyze the incoming audio, generate subharmonics and either inject them or send them to a dedicated output. Sounds like what you are looking for along with a custom sub designed to reproduce frequencies considered below the normal audio range.

https://dbxpro.com/en-US/product_families/subharmonic-synthesis