The ornamental design for a headlessuprightbass, as shown and described.
FIG. 1 is a front elevation view of a headless upright bass showing my new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view therof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the headless upright bass and form no part of the claimed design.