We propose and demonstrate a multistage design for microphotonic add–drop filters that provides reduced drop-port loss and relaxed tolerances for achieving high in-band extinction. As a result, the first microring-resonator filters with a rectangular notch stopband in the through port (to our knowledge) are shown, with extinctions exceeding $50\mathrm{dB}$ . Reaching $30\mathrm{dB}$ beyond previous results, without postfabrication trimming, such extinction levels open the door to microphotonic notch circuits for spectroscopy, wavelength conversion, and quantum cryptography applications. Combined with a low-loss, high-index-contrast electromagnetic design in $\mathrm{SiN}$ and frequency-matched microring resonators, this approach led to the first demonstration of flattop microphotonic filters meeting the stringent criteria for high-spectral-efficiency integrated add–drop multiplexers. The $40\mathrm{GHz}$ wide filters show a $20\mathrm{nm}$ free spectral range, $2\mathrm{dB}$ drop loss, and suppression of adjacent channels by over $30\mathrm{dB}$ .