  We explicitly check quark-hadron duality to order $(m_b-m_c)\Lambda/m_b^2$
for $b \to c l\nu$ decays in the limit $m_b-m_c \ll m_b$ including ground state
and orbitally excited hadrons. Duality occurs thanks to a new sum rule which
expresses the subleading HQET form factor $\xi_3$ or, in other notations,
  $a_+^{(1)}$ in terms of the infinite mass limit form factors and some level
splittings. We also demonstrate the sum rule, which is not restricted to the
condition $m_b-m_c \ll m_b$, applying OPE to the longitudinal axial component
of the hadronic tensor without neglecting the $1/m_b$ subleading contributions
to the form factors. We argue that this method should produce a new class of
sum rules, depending on the current, beyond Bjorken, Voloshin and the known
tower of higher moments. Applying OPE to the vector currents we find another
derivation of the Voloshin sum rule. From independent results on $\xi_3$ we
derive a sum rule which involves only the $\tau_{1/2}^{(n)}$ and
$\tau_{3/2}^{(n)}$ form factors and the corresponding level splittings. The
latter strongly supports a theoretical evidence that the $B$ semileptonic decay
into narrow orbitally-excited resonances dominates over the decay into the
broad ones, in apparent contradiction with some recent experiments. We discuss
this issue.
