We present a single frequency ytterbium-doped fiber amplifier, counter-propagation pumped with an all-fiber pump and signal combiner. The fiber combiner is capable of handling 440 W of pump power as well as the maximum achieved output power of 301 W. The threshold of stimulated Brillouin scattering of the high-power fiber amplifier was shifted from 175 to 280 W by external thermal management in combination with the pump-induced intrinsic temperature gradient.