Interesting comments, especially that you noticed a big difference between the Foil HMF and HMX. I'm always reading that the ride of these bikes is the same and that the HMX layup simply reduces weight for the same ride feel, but I suspect not many people have actually ridden both. How would you describe the difference between the Foil HMF and HMX?
Regarding stiffness and liveliness/suppleness, I've always believed that there is no magic here. A less stiff bike will feel more supple and lively as long as it's not /too/ flexy. Maximum stiffness isn't always best, especially for lighter riders. There may be an ideal level of stiffness for a given rider depending on weight and riding style, and/or preferred feel. But I'd put money on the differences between the ride feel of the Foil and the Addict being as simple as that - the Foil is a bit stiffer, so the Addict will feel more lively for some riders and not stiff enough for others.
I have the Foil HMX incidentally.