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Evolution of stimulated Raman scattering (SRS) with simultaneous seeding of amplified spontaneous emission (ASE) and signal pulse on a forward pumped high power Ytterbium-doped double-clad fiber amplifier is studied. A cascaded amplifier chain configuration is used for the study. It is shown that the generated Raman-stokes pulses occur together with the signal pulse causing optical output signal pulse distortions. It is also experimentally demonstrated that increasing the ASE seeded power the energy contained in the first-order Stokes Raman pulse can be significantly reduced. The seeded ASE changes the gain dynamics of the high power amplifier stage, reducing SRS and having a strong impact on the enabled extracted energy within the signal pulse. The signal and Raman spectral components are discriminated and the temporal evolution of each component for different input ASE power levels is analyzed individually. Quantitative results of the energy transferred from the signal to the Raman pulse can in this way be accurately obtained.