also this may help
http://www.soundonsound.com/sos/jan01/articles/advanced.asp
here is the key text to read
Compressors are often used in conjunction with equalisers, especially in mastering applications. However, there's a big difference in the results achieved, depending on whether you put the EQ before or after the compressor, especially if the compressor is a full-band type. Let me give you an example: let's assume that a mix needs more low-end energy, so we add some bass boost at 80Hz. If we then feed the EQ'd signal through a compressor it will respond most to the loudest signal peaks, which in all probability will occur exactly where we applied the boost — in other words the compressor will attempt to turn down the level of the sounds we've just tried to emphasise. Sometimes this will produce a musically useful effect, but where you want the EQ to be unaffected by the compressor, you're better patching the compressor first in line. If you have a hardware EQ and a compressor, or corresponding plug-ins, I'd recommend you try a few experiments to demonstrate just how great a difference can be made simply by moving the EQ before or after the compressor.