The restored strain energy in PbZn1/3Nb2/3O3 (PZN) and 9%PbTiO3-doped PZN (PZN-9%PT) relaxor crystals has been studied by means of acoustic emission (AE). Two types of AE activity signals have been recorded: (i) related to temperature- or electric-field-induced macroscopic phase transitions and (ii) associated with formation/disappearance of intrinsic polar nanoregions. Monitoring of AE under varying [001] electric fields has allowed a unique in situ observation of a low-field (1 kV/cm) irreversible orthorhombic-to-M-C phase transition within the morphotropic phase boundary region of PZN-9%PT.