Choose foam for sofa seat cushion replacement

[vdog1]

Help needed selecting foam for my first DIY project - replacing old cushion foam on my sofa.

After 12 years, my sofa cushions have gotten too soft and need to be replaced (fabric and structure are fine). Since I'm retired and have the time I'm going to tackle this myself. There are three boxed cushions that will each need a total thickness of 6" of foam wrapped in Dacron. I'm the only one sitting on the sofa, and weigh about 125 lbs (so don't break in cushions too quickly). I want something immediately comfortable, not too soft but definitely not too firm (I've had to return sofas in the past that were just too hard).

Looking for foam advice - should I do one 6" block of 2830 or 1828 or 1833 or 1835?

Or should I do a 4" block of that foam with a 1" layer on each side of something softer (2823 or 1824)? Everything I see in the stores these days has multiple foam layers, not just one block - I'm not sure if that helps with comfort, vs. using just one 6" foam core.   

Live in greater Denver - Thinking of buying foam online from Ronco or the Foam Factory....

Thanks so much for any advice.