  We report $^{93}$Nb and $^{121}$Sb NMR and $^{57}$Fe M\"{o}ssbauer studies
combined with DFT calculations of Nb$_{1-x}$Ti$_x$FeSb ($0\leqslant x
\leqslant0.3$), one of the most promising thermoelectric systems for
applications above 1000 K. These studies provide local information about
defects and electronic configurations in these heavily $p$-type materials. The
NMR spin-lattice relaxation rate provides a measure of states within the
valence band. With increasing $x$, changes of relaxation rate vs carrier
concentration for different substitution fractions indicate the importance of
resonant levels which do not contribute to charge transport. The local
paramagnetic susceptibility is significantly larger than expected based on DFT
calculations, which we discuss in terms of an enhancement of the susceptibility
due to a Coulomb enhancement mechanism. The M\"{o}ssbauer spectra of
Ti-substituted samples show small departures from a binomial distribution of
substituted atoms, while for unsubstituted $p$-type NbFeSb, the amplitude of a
M\"{o}ssbauer satellite peak increases vs temperature, a measure of the
$T$-dependent charging of a population of defects residing about 30 meV above
the valence band edge, indicative of an impurity band at this location.
