Don't recall any ill effects with them stacked... Just make sure there's space for airflow for the amps to stay cool. If you hear hum, move stuff (but you won't with line level stuff because hum breakthrough indicates less than great design). You won't damage anything through positioning.
Ignore the bull about demagnetising CDs etc. it's just another way of taking money off 'audiophiles' with no technical knowledge - like green pens, charged ball bearings etc.