Sub-synchronous torsional instability, which may arise from interactions between synchronous generators and power electronic devices, has traditionally been evaluated through electrical damping calculations derived from electromagnetic transient simulations. However, the increasing integration of power electronic devices associated with the energy transition has significantly increased the complexity of such assessments, rendering them computationally demanding. To address this challenge, this paper applies a system-level immittance-based frequency-domain approach for torsional stability assessment in multi-converter power systems. A brief review of conventional electrical damping and frequency-domain immittance-based methods is presented. Both methods are subsequently applied to a compact yet representative power system consisting of a synchronous generator, series-compensated transmission lines, and multiple power electronic devices to investigate torsional resonance phenomena. In addition, an eigenvalue-based approach is proposed to establish performance requirements for power electronic devices and to support the mitigation of torsional resonance instability. The results demonstrate that the proposed approach provides stability assessments that are consistent with those obtained from conventional electrical damping calculations while offering substantially improved computational efficiency. The validity of the proposed approach is confirmed through electromagnetic transient simulations.