Reflectionless low-pass, high-pass, band-pass, band-stop, all-pass, and all-stop filters, as well as a method for designing such filters is disclosed, along with a method of enhancing the performance of such filters through the use of sub-networks to further modify and improve the frequency response. These filters preferably function by absorbing the stop-band portion of the spectrum rather than reflecting it back to the source, which has significant advantages in many different applications. The sub-networks preferably offer additional degrees of freedom by which the leakage through the parent filter may be cancelled or reinforced to alter cutoff sharpness, stop-rejection, or other measures of performance.