From classical to modern ether-drift experiments: the narrow window for a preferred frame

Modern ether-drift experiments look for a preferred frame by measuring the difference \Delta \nu in the relative frequencies of two cavity-stabilized lasers, upon local rotations of the apparatus or under the Earth's rotation. If the small deviations observed in the classical ether-drift experiments were not mere instrumental artifacts, by replacing the high vacuum in the resonating cavities with a dielectric gaseous medium (e.g. air), the typical measured \Delta\nu\sim 1 Hz should increase by orders of magnitude. This prediction is consistent with the characteristic modulation of a few kHz observed in the original experiment with He-Ne masers. However, if such enhancement would not be confirmed by new and more precise data, the existence of a preferred frame can be definitely ruled out.