Hi JMB,
Thanks for the feedback! Personal experiences really add to the value of the information in the forum 
I talked with PLB this morning (she was actually the PLB rep for Healthy Back) and she confirmed that this must have been a floor model with the “old” construction. I also agree with you about the quality of the staff at Healthy Back and I have talked with several of their outlets and always been impressed with their knowledge and willingness to help.
You will find this throughout the industry. Part of the reason for this is that mattresses and foundations are tested together for the burn test to pass the fire code and part of this is that many outlets want to sell the foundation with the mattress. Some manufacturers will even make a particular foundation a requirement for their warranty when all that is really needed is a “suitable” foundation. In some cases an innerspring is an active part of the design of a sleeping system and is part of the design but in most cases a suitable “generic” innerspring or firm semi flex or non flexing foundation (depending on the type of mattress) is all that is needed.
This would depend on the actual layers of the mattress and on what comes with it (such as a foundation). The PLB Essence is certainly a better than average value when compared to anything like it in a mainstream mattress (it would compare to the PLB Nature if you did a google search but it has a Celsion upper layer though which is a more premium and cooler sleeping version of Talalay latex) but it may be more than many comparable mattresses made by local manufacturers.
The Diamond Ethos is probably better “value” ($2699 for the set). It uses less latex but includes the foundation and also uses wool quilting which is an expensive material. They also use inner tufting instead of gluing the layers together which can be used to affect the feel of the mattress (it compresses the materials) even though the glue used in most high quality latex mattresses that use it (including PLB) is non toxic. This is just a materials comparison though between “good and good” mattresses and even more important is how well each mattress “fits” you for Pressure relief, Posture and alignment, and your Personal preferences about how it feels when you lie and move on it (with your partner if you have one). Over the course of a decade (or likely more with a higher quality mattress) … the difference in price between two high quality mattresses may not be nearly as important as how well the mattress works for you. A mattress has a bigger effect on your overall wellbeing than any other furniture purchase you will make and this is also a big part of the value of a mattress for each individual.
There is no doubt about the quality of both of your choices but as you mention $3000 is a lot for mattress only, even for a King size that uses high quality materials. I would make sure you were happy with every aspect of the mattress and that you had compared it to similar mattresses both locally (on the phone now that you know the basic construction you are happy with) and online to make a few “value comparisons” to make sure you are happy with the price.
You mentioned in an earlier post …
I would make a few comparisons to some of the latex mattresses offered in post #21 here and use them as a “value reference” (value in terms of materials/price).
I would also talk with (and depending on the outcome of the conversation visit as well) Los Angeles Adjustable Beds Phoenix Latex Mattress if you haven’t already and I would also consider giving https://www.foamsweetfoam.com/ a call as well (they are on the Los Angeles list but are in Lake forest and may be worth talking to as well).
These are to help you in your “bargain hunting” so you have better value references and perhaps some “ammunition” for “making an offer”.
Phoenix