Without knowing the original signal it is hard to guess why you don’t see a big difference. As you mentioned, the original signal might already be filtered (EEG amplifiers often have hardware filters). In addition, if your goal is to suppress 60Hz line noise, the edge frequency of 55Hz might be too close to yield meaningful attenuation. The discussion in band pass filter and notch filter for an upper frequency do not give the same result might also be relevant.
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