We propose in this paper a 4-channel time-domain test bench for the characterization and linearity enhancement of microwave power amplifiers (PAs). The proposed time-domain measurement system utilizes four track and hold amplifiers (THAs) for direct subsampling of radiofrequency (RF) signals. The use of wideband THAs to replace samplers or mixers enables reducing analog IF circuit complexity. It permits direct digitization of RF signals like CW, two-tone and pulsed modulated signals, bringing more flexibility in the receiver's performance by enhancing the dynamic range and bandwidth. This test bench is capable of completely extracting the phase, amplitude, and transfer characteristics of non-linear devices excited with CW or modulated RF signals. In this work, two-tone transfer characteristics of a 50 W GaN HEMT Nitronex PA were extracted and processed for applying digital pre-distortion linearization to enhance linearity performance. Time-domain envelope and carrier waveforms (voltage and current) along with third-order inter-modulation distortion (IMD) products with and without digital pre-distortion scheme are also presented.