The Collaborative International Dictionary of English v.0.48:

Sub-base \Sub"-base`\, n. (Arch.)
   The lowest member of a base when divided horizontally, or of
   a baseboard, pedestal, or the like.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sub-bass \Sub"-bass`\, n. (Mus.)
   The deepest pedal stop, or the lowest tones of an organ; the
   fundamental or ground bass. [Written also sub-base.]
   --Ayliffe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subbase
    n 1: the lowest molding of an architectural base or of a
         baseboard