  Using a large sample (~11800 events) of D^+ into K^- pi^+ e^+ nu_e and D^+
into K^- pi^+ mu^+ nu_mu decays collected by the CLEO-c detector running at the
psi(3770), we measure the helicity basis form factors free from the assumptions
of spectroscopic pole dominance and provide new, accurate measurements of the
absolute branching fractions for D^+ into K^- pi^+ e^+ nu_e and D^+ into K^-
pi^+ mu^+ nu_mu decays. We find branching fractions which are consistent with
previous world averages. Our measured helicity basis form factors are
consistent with the spectroscopic pole dominance predictions for the three main
helicity basis form factors describing D^+ into anti-K*0 ell^+ nu_mu decay. The
ability to analyze D^+ into K^- pi^+ mu^+ nu_mu allows us to make the first
non-parametric measurements of the mass-suppressed form factor. Our result is
inconsistent with existing Lattice QCD calculations. Finally, we measure the
form factor that controls non-resonant s-wave interference with the D^+ into
anti-K*0 ell^+ nu_mu amplitude and search for evidence of possible additional
non-resonant d-wave or f-wave interference with the anti-K*0.
