A block of mass \(m\) on a horizontal surface is attached to an ideal spring with spring constant \(k\), where \(x = 0\) represents the spring's unstretched equilibrium position. The coefficient of kinetic friction between the block and the surface increases with position according to \(\mu_k(x) = bx\), where \(b\) is a positive constant and \(x > 0\). The block is pulled to position \(x = x_0\) and released from rest. Which of the following expressions represents the speed of the block as it first passes through the equilibrium position \(x = 0\)?