  We investigate a tadpole-improved tree-level-O(a^3)-accurate action, D234c,
on coarse lattices, using the mean link in Landau gauge to measure the tadpole
contribution. We find that D234c shows much better rotational invariance than
Sheikholeslami-Wohlert, and that mean-link tadpole improvement gives much
better hadron mass scaling than plaquette tadpole improvement, with finite-a
errors at 0.25 fm of only a few percent. We explore the effects of possible
O(alpha_s) changes to the improvement coefficients, and find that the two
leading coefficients can be independently tuned: hadron masses are only
sensitive to the clover coefficient, while hadron dispersion relations are only
sensitive to the third derivative coefficient. Preliminary non-perturbative
tuning of these coefficients yields values that are consistent with the
expected size of perturbative corrections.
