In this paper we consider a wave energy converter concept which is created by linking a box barge to the mechanical reference by linear dampers. The response to incident wave action in terms of power take-off is expressed explicitly as the solution of a linear frequency-domain model. The simplicity of the model combined with the possibility of the application of theory allows for a nested, and so manageable, procedure of optimization. We find that for any geometry, i.e., a combination of e.g. the breadth-to-length and breadth-to-draught aspect ratios of the box, the optimum is characterized by resonance at least in one of the two degrees of freedom, heave or pitch. Furthermore, optimal geometries turn out to be extremal: either long attenuator-type or wide terminator-type devices perform the best. We find also that optimal wavelengths, which are comparable to the device length in case of attenuators, emerge either due to the progressively increasing buoyancy restoring force characteristic, or due to the finite bandwidth of irregular waves. In particular, diffraction forces are more significant under optimal conditions for performance in irregular seas in comparison with conditions necessary for the most intensive displacement response of the free-floating box barge exposed to regular waves. (C) 2015 Elsevier Ltd. All rights reserved.