Sure, Big Fat Dots make a great interface between audio stand and floor. They adapt best to bare floors, though a thin carpet is no problem. You might also consider Giant Fat Gliders. These provide the decoupling and isoltaion of Big Fat Dots with the added benefit of easier mobility whenever you might wish to move the rack a little.
Tenderfeet usually do best in matched sets of four. They are not very much weight-specific, having a broad range of capacity. An audio component usually has vibrational character that's a factor of the unit as a whole--not as a consequence of how much weight is distributed to each footer. At forty pounds, amplifiers tend to favor being on Extra-Firm Tenderfeet (a CD player of the same weight usually favors TenderSoft Tenderfeet, though both will do very well with either the firm or soft Tenderfeet.
Herbie's Audio Lab