Utilising Hilbert-pair-based digital filtering, intensity modulation and passive optical coupling, optical filter- and O-E-O conversion-free ROADMs with excellent flexibility, colorlessness, gridlessness, contentionlessness, adaptability and transparency to physical-layer network characteristics are proposed and evaluated, for the first time, which offer DSP-enabled dynamic add/drop operations at wavelength, sub-wavelength and orthogonal sub-band levels. Extensive numerical simulations are undertaken to explore the operation characteristics of the proposed ROADMs in IMDD-based optical network nodes. It is shown that the add/drop operation performance is independent of a signal’s location in the digital filtering space. In addition, the results also indicate that the drop operation introduces negligible optical power penalties, whilst for the worst-case scenarios, optical power penalties induced by the add operation can be 3.5dB. Furthermore, the impacts of key digital filter parameters and intensity modulation-associated drop RF signals on the ROADM add/drop performance are also investigated, based on which optimum ROADM design criteria are identified for not only effectively reducing the digital filter DSP complexity but also simultaneously improving the ROADM performance.