Positioning Piezoelectric Actuators (PAs) are extremely versatile high precision investigative tools exploited in the field of micro-nanotechnology and they are also widely used in highly accurate industry production. PAs systems require appropriate controllers allowing to obtain fast and high-precision positioning performances but usually commercial systems provide integrated Proportional-Integral (PI) controllers providing standard stability performances also in the presence of uncertainty and disturbance. Those PI controllers usually are not tunable or adjustable on the considered plant, limiting controlled system performances with respect to nominal PAs features. In this paper the authors propose to improve the PI pre-compensated system performances through an external Linear Quadratic Gaussian (LQG) controller, in order to modify PIs reference signals behaviour by a feedback control law. The considered system is a micro-nano resolution triaxial piezoelectric driver controlled by a commercial amplifier providing a set of internal analog PI controllers. In this paper, the PI pre-compensated plant is identified in the form of a Multi-Input Multi-Output (MIMO) Linear Time-Invariant (LTI) model, due to the linear behaviour of the actuated axes. Effectiveness of the proposed method is validated by simulation tests and experiments on the real system, comparing the proposed approach with respect to the PI pre-compensated plant performances.