The ornamental design for a tubsupportbase, as shown and described.
FIG. 1 is a top, front, left side perspective view of a tub support base
showing my new design, the broken line showing is for illustrative
purposes only and forms no part of the claimed design;
FIG. 2 is a rear elevational view thereof, the front elevational view being
a mirror image thereto;
FIG. 3 is a right side elevational view thereof, the left side elevational
view being a mirror image thereto; and,
FIG. 4 is a bottom plan view thereof, the top plan view being a mirror
image thereto.