Thank you so much for your reply. After applying re-referencing prior to filtering, I observed a significant reduction in power at 25 Hz, 75 Hz and related harmonic frequencies. Subsequent 1–40 Hz bandpass filtering further yielded a remarkable improvement. While I am still puzzled by the underlying mechanism, this workflow has proven practical and highly effective. Your suggestions have been incredibly helpful.
I am nonetheless curious about why re-referencing caused such drastic changes. I conventionally perform re-referencing after ICA, mainly to account for bad channels and avoid rank deficiency issues during ICA computation.
Thanks again for your kind assistance.