An approach to analysis of nonlinear systems is proposed based on excitability index-fully nonlinear analogue of magnitude frequency response of the system. It can be used for stability analysis of fully nonlinear cascade systems similarly to absolute stability analysis of Lur'e systems. Excitability index evaluates ability of the system to exhibit resonance-like behavior under feedback action: feedback resonance. Speed-Gradient algorithms of creating feedback resonance in nonlinear multi-DOF oscillators are described. For strictly dissipative systems bounds of energy and excitability change by feedback are established.