Combined x-ray Brillouin scattering and electrical measurements were used to study acoustoelectric interactions in semidegenerate n-GaAs at 100 K. The measurements provided the frequency dependence of the lattice-loss parameter and a set of data for the acoustoelectric gain parameter for the  fast transverse acoustic wave in the range of frequencies of tens of GHz. The lattice-loss parameter was found to follow an ω1.1-1.3 dependence which is close to the linear dependence expected in the Landau-Rumer regime. The numerical values of the experimental lattice attenuation could not be approached by the usual approximations in this regime, a fact that stresses the need for a more complete calculation of this parameter in the hypersonic regime. Comparison of the experimental small-signal spectra and the acoustoelectric parameters with their theoretical predictions show that although quite good agreement is obtained in narrow frequency ranges for each concentration, discrepancies are noted in the high-frequency regime. These discrepancies are smaller when the spectra are calculated according to Mosekilde's quantum theory which takes into account carrier degeneracy. They are exceedingly higher when the semiclassical theory, based on Maxwell-Boltzmann statistics, is used. Although the studies in this paper were mostly confined to the small-signal regime, some results obtained in the large-signal regime are also presented. In this regime, the structure of the acoustic spectra exhibit the up- and down-conversion to high harmonics and subharmonics of the central amplified frequencies. The saturated spectra which could be measured only in several cases were broad, had no pronounced structure, and their maxima were downshifted from the small-signal peak frequency by only 30%.