Originally Posted by Lagerfeldt
It's the phase relations that change.
The original waveform (your mix) consists of a variety of frequencies that interact and constitute a whole.
When you use an equalizer you're not only changing the amount of gain in a particular frequency area, but you're also changing the phase and therefore the timing between the various frequencies.
This causes the various frequencies to interact in a different way that may or may not lead to a positive change (peak) as the frequencies shift - even though you're cutting. Remove enough though and you'll see the peak level go down again, but that's not the solution of course.
Most equalizers are so-called minimum phase and have this natural and normally desirable behaviour. Quite often when you're equalizing you're not just changing the amount of gain in a particular area, but you're trying to obtain the right phase response.
However, you can use a phase linear equalizer to offset the relative phase changes. In your case you'll observe almost no peak change, but still get the low cut you're looking for. Linear phase is no magic bullet though, and you'll probably hear some transient blurring caused by pre-filter ringing, which is an unwanted side-effect of linear phase technology. This is one of the reasons why linear phase is not the first choice for many engineers.
Logic Pro has a bundled linear phase equalizer, but you can also get the DMG Audio EQuality and switch between modes on the fly.
Lagerfeldt, you are an ME, I know the change in phase is a by product of EQ, Digital or Analog, but is there any difference in the way phase is changed by a digital Minimum Phase EQ vs by an Analog MP EQ?
From what I know there shouldn't be, but then maybe you or someone else knows something different.