A closed SL(3, C)-structure on an oriented 6-manifold is given by a closed definite 3-form ρ. In this paper we study two special types of closed SL(3, C)-structures. First we consider closed SL(3, C)-structures ρ which are mean convex, i.e. such that d(Jρρ) is a semi-positive (2, 2)-form, where Jρ denotes the induced almost complex structure. This notion was introduced by Donaldson in relation to G2-manifolds with boundary and as a generalization of nearly-Kähler structures. In particular, we classify nilmanifolds which carry an invariant mean convex closed SL(3, C)-structure. A classification of nilmanifolds admitting invariant mean convex half-flat SU(3)-structures is also given and the behaviour with respect to the Hitchin flow equations is studied. Then we examine closed SL(3, C)structures which are tamed by a symplectic form Ω, i.e. such that Ω(X, JρX) > 0 for each non-zero vector field X. In particular, we show that if a solvmanifold admits an invariant tamed closed SL(3, C)-structure, then it has also an invariant symplectic half-flat SU(3)-structure.