A high-power, single-frequency, ytterbium doped photonic crystal fiber amplifier using a Nd:YAG non-planar ring oscillator seed source is reported. The system delivers up to 148 W of output power and a slope efficiency of 76\% with respect to the launched pump power. At maximum output power the amplified spontaneous emission was suppressed by more than 40 dB and a polarization extinction ratio of 28 dB was obtained. Single transverse-mode operation with M2 values better than 1.2 was measured by a standard method. To investigate the real overlap of the photonic crystal fiber transverse-mode with the Gaussian fundamental-mode, sensitive beam quality measurements with a Fabry-Perot ring-cavity premode cleaner will be presented.