In contrast to ordinary ferroelectrics where the temperature T-m of the permittivity maximum monotonically increases with bias field E in (1-x)PbMg1/3Nb2/3O3-(x)PbTiO3 (0 <= x <= 0.35) single crystals, T-m was found to remain constant or to decrease with E up to a certain threshold field E-t, above which T-m starts increasing. The threshold field E-t decreases with increasing x, tending toward zero at approximately x=0.4. We explain this dependence in the framework of models which take into account quenched random fields and random bonds. For crystals with 0.06 <= x <= 0.13, the E-T phase diagrams are constructed. In contrast to PMN, they exhibit an additional, nearly field-independent boundary, in the vicinity of the Vogel-Fulcher temperature. We believe this boundary to correspond to an additional phase transition and the appearing order parameter is likely to be nonpolar.