We report on the successful implementation of an adaptive pre-amplification pulse shaping technique in a high-power, coherently combined fiber laser system to achieve sub-300-fs pulse durations at 320 W average power and 3.2 mJ pulse energy. The pulse shaper is utilized to impose a gain flattening mask to increase the spectral width of the amplified pulse by 60%. Simultaneously, it pre-compensates the spectral phase acquired in the multi-stage amplification and subsequent compression including the eight-channel, coherently combined main amplification stage. This result does significantly enhance the performance of the fiber laser system and the subsequent nonlinear compression stages.