This is an extra firm mattress. 10" thick from Beautyrest. 
A good night sleep is what a person needs after working hard. This mattress is sure to provide that. We are very happy with this mattress. My husband has backpain and this firm mattress is very comfortable for him and he says his backpain has considerbly reduced. I am sleeping well as you dont feel the other persons toss n turns on this.  
I am buying a king size mattress for my guest bedroom. 
No need to put a box under. Its very firm but if your bed requires one please use the box otherwise its not required.
A very big thank you from hubby to Simmons Beautyrest.