A lot of times it's more of a feel thing to...most of the time I can hear an amp thats SS rectified and/or heavily filtered but I can tell it when I play though one ALL the time...makes for a really differnet feel from a players stand point.
I like tube rectifiers on cleans and leads. On cleans it softens the edges of it so it's more pleasant and less abrupt. On leads it kind of smears and greases everything up and gives you more of that out of control, electric guitar feel.
On rhythm I prefer solid state rectifiers. They have a tighter, more solid bottom end and generally seem to have a little more top end detail.
Stiff = "tight" when you don't like the amp