The Englander Boston Extra Firm is a good option for those who prefer an unmistakable firm feel. It has a 10 year warranty.
Amerisleep AS4 Soft Mattress (Discontinued)
The Amerisleep AS4 Soft Mattress has a unique feel to the top layer where it feels fairly dense, but there is also a slight sinking feeling.
Amerisleep AS3 Medium Soft Mattress (Discontinued)
The Amerisleep AS3 Medium Soft Mattress feels quite dense and firm, but it seems to mould to the body quite well, still making it comfortable. As such it maintains body position well, and spinal support is very good.
Amerisleep AS2 Medium Firm Mattress (Discontinued)
The Amerisleep AS2 Medium Firm Mattress has a dense, foamy feel which is quite firm and offers very good spinal support. Partner disturbance is minimal.
Amerisleep AS1 Firm Mattress (Discontinued)
The Amerisleep AS1 Firm Mattress is the firmest version in the Amerisleep range. As you would expect, it feels firm and dense.