24dB/octave Linkwitz/Riley crossovers are indeed amplitude flat through the crossover point. This can be proven by summing the two sections (lo pass and hi pass) through two equal value resistors and measuring the frequency response through the crossover region. It will be ruler flat. The phase also has to be zero degrees too since any other phase angle would introduce amplitude variations in the crossover region with the simple summed resistor network, or any active summing netork.