The present work deals with tunable electrical interfaces able to enhance both the harvested power and bandwidth of piezoelectric vibration energy harvesters. The aim of this paper is to propose a general, normalized, and unified performance evaluation (with respect to the harvested power and bandwidth) of the various electrical strategies that can tune the harvester’s frequency response. By mean of a thorough analysis, we demonstrate how such interfaces influence the electromechanical generator response through an electrically-induced damping and an electrically-induced stiffness. The choice of the strategy determines these two electrical quantities, and thus the achievable frequency response of the system. Thereafter, we introduce a collection of graphical and analytical tools to compare and analyze single- and multi-tuning electrical strategies, including a qualitative performance evaluation of existing strategies. Finally, we establish a unified comparison of single- and multiple-tuning strategies which is supported by the definition and evaluation of a new optimization criterion. This comparison reveals which strategy performs best depending on the electromechanical coupling of the piezoelectric harvester and on the losses in the electrical interface. Furthermore, it quantifies the power and bandwidth gain brought by single- and multi-tuning strategies. Such quantitative criterion provides guidance for the choice of a harvesting strategy in any specific applicative case.