By incorporating an isospin-dependent form of the momentum-dependent potential in the ultra-relativistic quantum molecular dynamics (UrQMD) model, we systematically investigate effects of the neutron-proton effective mass splitting = and the density-dependent nuclear symmetry energy on the elliptic flow in + collisions at beam energies from 0.09 to 1.5 GeV/nucleon. It is found that at higher beam energies ( 0.25 GeV nucleon) with the approximately 75 MeV difference in slopes of the two different , and the variation of ranging from –0.03 to 0.03 at saturation density with isospin asymmetry , the has a stronger influence on the difference in between neutrons and protons, i.e., , than has. Meanwhile, at lower beam energies ( 0.25 GeV nucleon), is sensitive to both the and the . Moreover, the influence of on is more evident with the parameters of this study when using the soft, rather than stiff, symmetry energy.