Did they mention your ejection fraction? Perhaps if that is low and blood isn't circulating the way it should, you may experience some brain fog. My Dad is in end stage heart failure and he's been foggy for quite awhile (not to say you have heart failure, just that it can affect cognitive abilities).
I have not heard of a relationship between aortic valve stenosis and difficulty concentrating. However, if it's been awhile since your last echo, it's probably worth a call the doctor's office. I suppose there is a chance you are not getting an adequate blood supply. However, I also know (first-hand) that some of this is common as we age.
When was your last echo?