Hi, siava1018. I recommend using a Square Fat Dot under each of the cleats or legs. If the cleats are pointed rather than broad and flat, you'll want to use something else under them, though. Maybe Cone/Spike Grounding Bases.
Tenderfeet usually do best in direct contact with the component chassis. Thin wood and shims placed under the component as a false bottom will likely introduce some unwanted "character" and inhibit the Tenderfoot effectiveness. Tenderfeet, being free-standing, are very versatile with placement--you should be able to find locations under the scd where they'll support the component and keep it level. If not, individual Tenderfeet can be given a boost with Square Fat Dots, felt, or shimmed with sheets of Post-It Pad paper underneath.
A moderate-weight sandbox on top would probably be beneficial, separated from the component with grungebuster Dots or Extra-Thick grungebuster Dots. With compliant Tenderfeet underneath, you don't need a ton of mass-loading weight on top; just a moderate weight like placing the palm of your hand on the component to stabilize it. Setting the component in a sandbox provides very good isolation from the shelf. If using a sandbox underneath, place a fairly thick Baltic birch plywood plinth on the sand, then the component with four Tenderfeet on the plinth.
Rope caulk (available at hardware stores) is a good remedy for a flimsy/weak chassis. Place some broadly under the lid, along the sides and bottom, around circuit board and motor mounts. The material is sonically neutral and absorbs micro-vibrations superbly.
The tube buffer should do very well on four Tenderfeet or Baby Booties. For additional benefit, a SuperSonic Stabilizer or moderate-weight sandbox or sandbag on top should do well also. I doubt the added weight of a sandbox would transfer more vibration to the tube. An inappropriate choice of materials could be sonically detrimental, though.
To further improve isolate of the bookcase, employ isolation/decoupling between the loudspeakers and floor (Herbie's Gliders or Big Fat Dots for example). Besides improved loudspeaker performance, this reduces floorborne and wall-borne vibrations that can infiltrate the bookcase and components, especially if the bookcase is coupled to the wall.
Herbie's Audio Lab