Hi Frillyfilly and welcome to the Mattress Underground 
Congratulations on your new Cantwell Mattress!
Hi! I have a new mattress that’s causing a lot of lower back pain. It’s so much softer than expected. My back and hips are sinking too low. It almost feels hard though at the same time. Will changing the density of the 2" latex layer give me more support or are the comfort layers just too thick?
Sorry to hear you are not feeling comfortable on your new Cantwell mattress; you are correctin that it may just be a matter of a firmer comfort level. as you may already know, any mattress you find comfortable will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) - thanks for providing those! - and your PPP (Posture & alignment, Pressure relief, and Personal preferences). As a lower BMI sleeper, you have many more options available for suitable comfort and support.
IV 1090 mattress :
Organic Fabric, Organic Cotton Batting -
Fabric is made of organic cotton. The fabric is very thin, but if you are asking how thick the quilt is, the quilt is approximately 2 inches thick.
2" natural Talalay latex
The ILD of the topper that we use is a range from 11-16. Each latex topper varies a bit because it derives from sap from the rubber tree. The 11-16 ild range falls under the N1 category for firmness. N2 is the next level firmer, then N3, N4, and N5. N1 is what we use for toppers and then will go firmer upon request or when performing a firmness change.
2.75" pocketed quantum micro coil
-The pocketed coil extends the entire width and length of mattress. The micro pocketed coil is just over 2.75" tall.
1 1/4" latex lumbar
-The lumbar latex is now ½" with the 30 ILD. It is zoned. Larger holes(softer) towards the head/foot, smaller holes(denser) in the lumbar area.
368 Heavy duty coil Not Pocketed
-The 368 is about 6" tall.
There is a 1" 2034 foam as well as 2" of dense fiber.
Total is 17" mattress
Thanks for providing the specifications! To sum up your mattress is
(17") Iv1090 custom
-2" Organic Fabric, Organic Cotton Batting
-2" natural Talalay latex ild 11-16
-2.75" pocketed quantum micro coil
-1 1/4" latex lumbar 30 ild
-6" 368 Heavy duty coil Not Pocketed
-1" 2034 foam
-2" of dense fiber.
As you are a lower BMI you certainly do not need that much thickness and in some cases it may be counterproductive because the thicker a mattress is the firmer the layers that are used need to be to ensure adequate support. (You can read This Post on the effects of layers and thickness) but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress “as a whole” matches your specific needs and preferences in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) than it does on just the thickness itself. as @Phoenix says " The main benefit of a thicker latex mattress (or any mattress that uses similar materials) is that it can be more adaptable for heavier weights and multiple sleeping positions. It will compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights (or parts of the body). A thicker mattress can also be part of a specific design that requires it such as some types of zoning or layering that needs more layers to accomplish the design goal of the mattress. It can even just be a matter of preference rather than “need”.
This seems like a good quality mattresses, but as they do a lot of customization, details are light on their website as to specifics. the comfort layer is extremely soft, and along with the quilted cover, it’s likely you are ‘sinking’ into the top layer and bottoming out on the micro oil layer below causing your pains. it’s possible a more medium firm top layer could alleviate this. It would be a good idea to try some different latex firmnesses at Cantwell or even another local showroom, just to see how a firmer comfort layer feels for you. If it’s all all possible you might also try removing the microcoil layer (if it’s removable of course), see how your comfort layer feels without that - or even try laying on the 30 ILD latex layer, again if the other layers can be removed.
The firmness of your comfort layer is entirely subject to the preferences of the sleeper(s); in your case, as you don’t seem to have any additional support needs, so the microcoil layer may be a bit of ‘overkill’ - but if you enjoy the feel, that’s fine! As you only have one comfort layer exchange, we would suggest you try out a few different firmnesses of top layer to ‘dial-in’ the one most suitable for your needs. Please feel free to let us know how the exchange goes!
~ Basilio