hi. side sleeper 6’ 185lbs. have 3" firm, 3" med from sleeponlatex (apparently neither dunlop nor talalay? some proprietary blend) and two 2" plush dunlop (from avocado) - sorry don’t know ILDs for any. my shoulders are killing me with this setup, tingle and go numb after a few hours. any ideas what the culprit is? i’m not married to the all-latex idea - do you think 8" coils would be better? or ditch the 3" firm base and add a 3" soft talalay on top?
Hi oosh.
Welcome to our Mattress Forum! 
Even though it is not possible to pinpoint the exact cause of your shoulder pains, (assuming that this is not a chronic issue that needs to be checked with a healthcare professional) what you describe seems to indicate an inadequate comfort/support balance of all the layers that you can do well with. There may be multiple reasons for the “symptoms” you are experiencing and you would do well to find first their root cause before considering your further options. You are not saying for how long you tried this configuration but generally, this “killing” type of pain is the result of pressure points from a mattress that is too firm for your needs and that your shoulders are sinking in and bottoming out into the firmer 3" medium layer below resulting in your “terrible numbing and tingling pain”. It may be that you just need to add some thickness to your mattress and have a layer of Talalay that is more point elastic and feels softer in the same ILD or firmness.
On average, thicker mattresses will “act” softer for most people. As far as how thick this should be it is difficult to say but I’d be careful with adding too much thickness as this can also throw your spine out of alignment. Generally, you’d want to find something with “just enough” surface plushness to assist with contouring for your shoulders (and hips).
The position of your arm and your body type and profile also need to be considered (e.g. someone with broader shoulders or a more unusual profile may do well with some zoning). How far each part sinks in has to do with how much weight it carries and the surface area that carries the weight, Thin or narrow shoulders will sink in easier than thicker wider shoulders. Narrow more pointed hips will sink in easier than the greater surface area of the same area on the back. This is part of the reason that weight and body shape are so important and that they are matched to a mattress. The mattress also could have been zoned to be softer under the shoulders and firmer under the hips.
Before considering your next purchase and changing the base layer to a different type of material, it would be interesting to note the position of your arms for your side sleeping. Research has found that the most common reasons for shoulder, cervical and upper thoracic pains are connected with pillow issues (thicker/thinner or firmer/softer pillow) which could also affect pressure on the shoulders. You don’t want your head to be too elevated, or not elevated enough. This is especially important when sleeping upon your side, as cervical /upper thoracic issues tend to be pillow-related.
Something that you may find assistive while you fine-tune your mattress would be to sleep with a pillow up against your back, allowing you to lean back slightly when sleeping on your side, effectively allowing you to “roll” your shoulder slightly forward so that you’re not sleeping directly upon it. Also, consider using a body pillow or a thicker pillow in front of you, upon which you may place your free arm. This also will take some pressure off of both of your shoulders. Finally, don’t forget to reassess your pillow whenever you get a new mattress to make sure that it is the proper thickness to fill in the gap from the outside of your shoulder to your ear, helping to take that weight off of your shoulder joint as well.
You can also read about the effects of thickness in latex layers here and in this TMU article Putting The Layers Together.
And also you may find some more clues in these posts: Fine Tuning a mattress in Post #4 and Post #7 and also Post #7 here
Hopefully, some of those tips will help, regardless of your eventual mattress combination.
Phoenix