I think of the term warmth in contrast to the word clear.
U47 = Warm
M49 = Clear
Coles 4038 = Warm
Royer 122 = Clear (as far as Ribbons go)
U67 = In between Warm & Clear
Tape = Warm
Digital = Clear
MCI Tape Machine = Warm & Colored
Studer 827 = Clear with a touch of warmth
But something can also have warmth and clarity in equal parts - when the low mids are round and hazy and the upper mids and top end are defined. KM84s are both warm and clear. API preamps are both warm and clear. Rode K2s are neither warm nor clear.