Metal sleeves (some are spring-loaded) are sometimes placed around tubes to prevent radio frequency interference and/or to hold them in place in a hostile environment (and for international shipping). In home audio applications, the sleeves have negligible utility and sometimes add a bit of weird-sounding influence. You can safely remove them and install damping instruments in their place.Likewise, some components have little cages around each tube, to protect the tubes from errant thrown balls, children, pets, or to alter proximate electromagnetic waves. These cages can be removed and damping instruments placed on the tubes for significant sonic improvement.