  We review recent theoretical improvements of Monte Carlo event generators for
top-quark pair production and decay at the LHC based on the POWHEG method. We
present an event generator that implements spin correlations and off-shell
effects in top-decay chains described in terms of exact matrix elements for
$pp\to \ell^+\nu_{\ell}\, l^-\bar{\nu}_{l} \,b \,\bar b$ at order $\alpha^4
\alpha_s^2$, including full NLO QCD corrections and interference effects with
single-top and non-resonant topologies yielding to the same final state. We
then compare its predictions to previous generators that implement NLO
corrections only in the top-pair production dynamics. We consider the mass
distributions of the $Wj_{\rm B}$ and $\ell j_{\rm B}$ systems, proxies for
direct top-mass determinations, and jet-vetoed cross section, a probe of the
$Wt$ single top contribution.
