There is considerable weight-capacity overlap between TenderSoft and Extra-Firm Tenderfoot isolation feet. 40 pounds is a general mid-point of their overlapping capacities, not a discrete cut-off. (Extra-Firm Tenderfeet do very well with components weighing much less than forty pounds; with very heavy components, TenderSoft version will squish down more than optimally thus losing some of its contra-vibrational energy.)
Weight considerations are based on four footers under the component. Four virtually always provide a sonically better result than using only three. Weight recommendation also takes into consideration that weight distribution of components is almost always uneven, especially with amplifiers, so individual weight-capacity is considerably greater than if you just divided a component's weight by four.
Additional Tenderfeet will increase the weight-handling ability and is oftentimes additionally beneficial sonically, for example placing a fifth footer under a motor mount, circuit board or other sensitive location, or simply placing one under the middle for a more whole-chassis vibration control.
Herbie's Audio Lab