We report a three-channel, spectrally beam-combined (SBC), $1\mu \mathrm{m}$ fiber laser that produces $522\mathrm{W}$ of average power with near-diffraction-limited $(M^2\sim 1.2)$ beam quality. The laser features a SBC power combining efficiency of 93%, versatile master-oscillator, power-amplifier fiber channels with up to $260\mathrm{W}$ of narrowband, polarized, and near-diffraction-limited output that is tunable over nearly the entire $1\mu \mathrm{m}$ ${\mathrm{Yb}}^{3+}$ gain bandwidth, and excellent prospects for significant power scaling. To our knowledge, these results represent the highest beam quality and average power achieved to date for a beam-combined fiber laser system.