Magnetic high-entropy alloys (HEAs) can be processed to have a relatively high saturation magnetization (or induction), but usually exhibit a relatively high coercivity of above 200 A/m. In addition, although a high electrical resistivity of magnetic HEAs makes them attractive for manufacturing energy-saving high-frequency magnetic devices, the critical application-oriented magnetism, i.e., the frequency- and induction-dependent power loss, of magnetic HEAs is largely unknown. Here, we report the (Co30Fe45Ni25)1-x(Al40Si60)x HEAs with exceptional mechanical, electrical, and magnetic performance. The magnetic (Co30Fe45Ni25)0.8(Al40Si60)0.2 HEA has a single-phased body-centered-cubic (BCC) structure, a high yielding strength of 1,636 MPa, and a high electrical resistivity ( R ) of 68.0 μΩ cm. This magnetic HEA also has the best combination of induction and coercivity reported so far: a high saturation induction ( B s ) of 1.24 Tesla and a low coercivity ( H c ) of only 59.7 A/m. Furthermore, the dependence of total power loss on both frequency and induction is investigated. The contribution of hysteresis loss, eddy-current loss, and abnormal loss to the total power loss is also studied. The abnormal loss is comparable to the eddy-current loss in both Fe-Si and magnetic HEAs. For a frequency above approx. 200 Hz, the total power loss of the (Co30Fe45Ni25)0.8(Al40Si60)0.2 HEA is lower than that of conventional Fe-Si alloys. This trend is mainly because the eddy-current loss of HEA is lower than that of conventional Fe-Si alloys.