The paper reflects the recent experimental results on the elaboration and study of active materials for magneto-acoustic phase conjugators (MAPCs). The results of complex measurements of MAPC parameters are demonstrated on typical samples of NiFe2O4 magnetostrictive ceramics. The mechanism of strong dispersion of gain increments and output power of MAPCs is studied and explained by dispersion of critical values of the parametric modulation depth of sound velocity. A maximum output power 240 W at frequency 5 MHz is obtained for active element MAPC with critical current Ic=9 A and electrical Q-factor equal to 80.