Recent work has demonstrated that the classical guitar can be advantageously augmented using a pickup to drive an actuator mounted on the guitar's back plate, thereby allowing enrichment of the instrument's timbral palette with audio effect processors in the loop. The feedback problem that results from such setup is similar to that occurring in live music performance setups where the sound of a guitar is amplified using a loudspeaker. In the case at hand measurements of the augmented guitar's open loop response demonstrate that instabilities are susceptible to first occur from the string's modes and not from the guitar's soundbox. In particular, the shape of the magnitude response suggests frequency shifting as a viable solution to string instability. Introduction of an upwards frequency shift in the forward path is proposed as a means for stabilizing the closed loop system. Experimental results demonstrate that the proposed solution leads to improved stability even for a modest frequency shift of 3 Hz. The achieved gain margin improvement, which is shown to be of at least 3 dB, then comes at the cost of a clearly perceptible amplitude modulation, which may be acceptable in conjunction with other audio effects chosen by the performer.