awesome, thanks for the link. I will now read through that big ol thread and report back here if there's any info as to WHY the amplitude increases, and where the audio energy gets shuffled to/from.

I actually edited my post to add some more information:
http://www.kvraudio.com/forum/viewtopic.php?p=5260046#526004 6

Hope, that helps.

This is a great topic. I had the same question myself when i was trying to remove low end to increase headroom in my mix, and it ended up increasing my amplitude!

I wonder if an true analog filter would have the same effect?

Of course.
They work after the same principles.

no,its not true /or it must be extremely rare,because Im using LPeqs every day more than 6 years and so far such situation never happened to me/

edit-the only situation imaginable where it can happen is with some electronic drums...here it can be possible as this is the only instrument I cannot withstand so cannot verify...

edit2-luckily I have found some old audio demo with electronic kick drum only I used for some compression testing and I have to accept that you were right...to some degree /but not more than cca +0.2 db peak level increase - which shouldnt be a big problem/

so the final answer for you: its probably because of filters resonance as LP eqs exhibits zero phase shift...

yes, totally true! see for yourself. fabfilter pro-q and dmgaudio equality, both in linear phase mode. they each add about 5db of gain when the cutoff gets to 1khz, and it goes down a bit when I move the cutoff up to 3khz. This break is using Impact Soundworks Groove Bias acoustic drum samples, processed a bunch.

http://www.youtube.com/watch?v=8RJXN3q4e-c

linear phase eq exhibits linear phase shift
zero phase has zero phase shift

well,since Im not any dsp expert I have to believe what LP eq dev claims:

Conceived and created with Mastering in mind, Linear Phase EQ preserves your original musical balance while providing precision control over the harmonic spectrum with absolutely zero phase shift.

Read more: http://www.waves.com/Content.aspx?id=207#ixzz2LrS3u4iB