When my AS got to the severe stage last summer, I was still walking 3 miles per day or more with no symptoms and kept my heart rate at 120 or below. Doc allowed that grudgingly but when I jokingly talked of "running" in a 10k or some such he looked at me and said "I would not run anywhere with that valve.
It all depends on the level of stenosis, and the heart wall thickening. You know that the mode of failure for AS is somewhat different than other heart related stuff. The wall thickens as the heart compensates(or tries to) for the narrower opening at some point(usually as a result of exertion) the ventricle dilates and quits or as my cardio puts it technically it "poops out". AS can lead to sudden death with this mode of failure. So, symptomatic or not it seems that exerting to hr=>160 is a form of "Russian Roulette" but I am not a doc and don't pretend to have all the answers. I just know I took seriously this cardiologist when he cautioned me against it regardless of how good I felt and how active I was. The echo numbers were telling the tale. BTW my cross sectional area was down to .61 cm sq. peak gradient at 80 only a slight thickening of the wall and NO symptoms at exercise. Do I think I was a canidate for surgery, only because of the numbers, not because of my perceived health.
Good luck,