The two-dimensional (2D) electron energy relaxation associated with acoustic phonon emission in GaAs/Gal(1-x)Al(x)As multiple quantum wells (MQW) has been investigated using hot-electron Shubnikov-de Haas (SdH) effect measurements performed at three different base lattice temperatures T-L0 congruent to 1.7, 3.5 and 5.9 K. The modulation-doped MQW samples studied have quantum well widths L-Z = 51, 75 and 78 Angstrom, and only the lowest subband in each sample is populated with a 2D electron density of about 1.10 x 10(16) m(-2). The electron temperature (T-e) has been determined from the lattice-temperature and electric-field dependencies of the amplitude of the SdH oscillations. The energy-loss rates show a power-law dependence on T, with an exponent gamma, which depends on T-L0. The experimental results are compared with the current theoretical models for power loss in 2D and 3D semi conductors, which include both piezoelectric and de formation-potential scattering. The electron-temperature dependence of power loss, determined experimentally at liquid-helium temperatures with T-L0 congruent to 1.7 K, fits well to both the 2D and 3D theoretical models in the low-temperature regime, while the results at T-L0 congruent to 3.5 and 5.9 K fit best to those in the intermediate-temperature regime. The results provide useful information about the relative magnitude of the deformation-potential and piezoelectric contributions to power loss.