I have noticed this several times before and I have never been able to work out why this should happen. If I apply a HPF to a sound the output after the EQ is sometimes noticeably higher that it was going into the EQ.
Today I have been working on a sound that peaks at 0dB FS (not one I recorded), it has a lot of low frequency content. I am applying a HPF at around 100Hz, with a 24dB per Oct slope. The output from the EQ is peaking at around +4dB FS on the meters within Logic. I am working quickly and so using the standard Logic EQ< I haven't tried this with my PSP EQ's to see if they exhibit the same behaviour, may try to experiment when I have more time.
Can anyone tell me if the same phenomenon occurs with analogue EQ? And most importantly, why should this happen, as most of the power in this sound is in the low frequencies, and it has little going on above 2KHz, so surely with the HPF, most ofthe energy from the sound has gone!