Hi Mehta,
With your wife's shoulder issues and your lower back issues and bulging disc, the best combination of comfort layer thickness and softness and the firmness of the support layers would be very important. While I don't know the exact construction that would end up working best for you (only your own testing on specific mattresses can know exactly) ... a good starting point would probably be in the range of 3" of softer latex over firmer support layers. The latex comfort layers would probably be in the range of 19 - 24 ILD depending on what you perceive as "firmer" and the support layers could be either latex, an innerspring, or polyfoam (depending on whether you wanted to reduce your budget and still have latex in the upper layers). The support layers need to be firm enough to keep your heavier parts from sinking in too far and putting your spine out of it's natural alignment in any of your sleeping positions while the comfort layers need to be soft and thick enough to allow shoulders to sink in enough to relieve pressure on the joints and pressure points of the body and also to fill in the gaps in your sleeping profiles.
While some people can "sleep on anything" without stress and pain and their testing only needs to be very approximate, when there are more sleeping issues to be dealt with, the ability to make custom adjustments to either layer thickness or layer firmness levels can be important if the models that are on the floor don't have a wide enough range of constructions and layering to find one that "fits" or are not perfectly suitable for your needs and preferences.
As you probably know ... the choice of outlet can be the most important first step in choosing a mattress and you are fortunate that there are several very good options near Charlotte, NC (3 right there and one within an hour) so it would probably be simpler and a good idea to wait till you were there. They are ...
www.lakemattress.biz/ Factory direct manufacturer in Conover. I have talked with Todd (one of the owners) on several occasions and they are transparent, knowledgeable and helpful. They make a range of memory foam, latex, and innersprings and have some very good quality and value options. They can also layer the materials they use in any of their mattresses at their factory so you can get a good sense of how they feel as well.
dilworthmattressfactory.com/Our_Location.html Charlotte. Local factory direct manufacturer. I have talked with Alan the owner and he makes a range of mattresses of all types (except memory foam) and is both knowledgeable and helpful. Can make custom adjustments to any of his mattresses to fit them to any individual. Good quality and value.
www.originalmattress.com/locations Charlotte area. Regional factory direct manufacturer that makes a full range of mattresses including traditional innerspring/polyfoam (including 2 sided), latex, and two memory foam mattresses with good quality and value. They tend to put all their mattresses on an "active" flex boxspring so if you are testing their memory foam or latex models, l would make sure you also test them on a firm foundation (such as an adjustable bed) because this can make a significant difference in how a foam mattress feels.
denvermattress.furniturerow.com/dmc/ Charlotte, NC. Regional factory direct manufacturer that makes a range of mattresses including innersprings and their iChoice which is a "mostly latex" mattress with either a memory foam or latex topper and several all latex mattresses as well. They are transparent and disclose all the information about the materials they use in their mattresses and are generally good quality/value choices compared to most mainstream brands. I would avoid the major brands they also carry.
www.sleepeasymattressgaffneysc.com/ Local factory direct manufacturer in Gaffney, SC. They make all types of mattresses including innerspring, memory foam, and latex.
www.theorganicsleepshop.com/ Retailer in Pineville that carries Savvy Rest component latex mattresses along with Organicpedic (OMI), NJaturepedic, Hypnos, and Gold Bond mattresses. They are all high quality mattresses but some of them may also be in more premium price ranges so I would make some careful "value" comparisons here.
At Original mattress I would suggest you include the Latex Supreme on a firm foundation (not just on their flexing boxspring) in your testing. At Dilworth they can make custom adjustments to their standard models so you are not limited to the exact layering of the mattresses that they have on display as they can be customized to your needs and preferences. At Myluxurymattress / Lake Mattress they have a fairly wide range of good choices to choose between.
Hopefully this will help and feel free to post with any questions if you need to.
Phoenix