Hi bdmma.
Welcome to our Mattress Forum! 
[quote]My favorite mattress I’ve had was a Serta Hybrid
But it seemed to be ideal in terms of firmness with just enough cushion to not hurt my hips when I sleep on my side. I did find it started sagging after a year or so, but it was still comfortable.[/quote]
Unfortunately Serta doesn’t fully disclose the type and quality of the materials in their mattresses and also tend to use lower quality and less durable materials in their comfort layers as well (See this video here of one of the TMU Expert member Arizona Premium of the site) and unless you are able to find out all the information you need to make an informed choice about a mattress (see this article) I would avoid trying to match it based on design (see the guidelines here ).
Unless the manufacturer/retailer that you are considering purchasing the mattress from is familiar with Serta Hybrid and attempts to match the comfort/support you enjoyed before the mattress failed and started sagging there is little value in trying to approximate the performance you got used to in your old mattress. This is certainly possible if manufacturer has enough experience and knowledge designs and builds a mattress that is reasonably close to the softness and support of another mattress that is known to them and this is confirmed with their own personal testing (and preferably the testing of a larger group of people like their customers as well) but you have many other options available to ensure suitability that I’d be tempted to suggest that you make a “clean break” with your old mattress and start anew looking for what you really prefer and need in a mattress that would last you for 10 years or so.
Post #10 here has a step by step process that can dramatically increase your odds of finding your “ideal” mattress that has better quality and value than anything you are likely to find with a major brand or typical mass market outlet.
I think you mean Mausund mattress which is an 8" medium-firm Dunlop mattes, but I I can’t predict what type of mattress you might do best with via an online forum, as there are entirely too many personal variables involved. Questions like yours get asked quite often on our forum and while I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see post #2 here ).
Again I can’t predict how a certain mattress, material or layering may work for you but Fleep is a 10" reversible memory foam/poly mattress with 2 firmness choices (firm and soft) and you can read some of my comments about Fleep mattress here and also some postings that discuss Silk &Snow here, and here that might be helpful to you.
If you’re looking for something using polyurethane foam, make sure that it has at least CertiPUR-US certification for the foam. If you desire something more “natural”, but want to keep the cost as low as possible, there are many options out there, but I’m most familiar with some offered by our Trusted members. This being said Latex is a very good and durable material and I wouldn’t hesitate to recommend it to you once you’ve tested it and determined that you like its feel.
The first place to start your research is the mattress shopping tutorial here (there is a condensed version of it at the end of that post) which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for)
Once you’ve had the chance to read through the information I have provided and do some local testing to narrow your choices feel free to get back to us and I or any of our Experts Members of the site will be happy to assist you.
Phoenix