When preparing your mixes for a mastering session, we typically get the question, "Should I take the L2 and compressor off the mix bus?" My first thoughts run through my head like asking why or what are you doing. . . but I refrain myself. I go on to explain that mastering engineers basically have 2 tools at their disposal: compression and eq. If the mixes are already compressed within an inch of their life, it ties one of the mastering engineer's hands behind their back. Also, I tell them if the compression is an integral part of the mix, should keep it in or you could bring both versions to the session. There are also many downsides to mixing with a heavy compressor on the mix bus. It hides imbalances in the mix and makes it very difficult to mix precisely. Frequently, engineers mix a record with a compressor on the mix bus all the time and just before sending it to mastering take it off as the last step. They find their mix falling apart and nothing like they have been crafting for the weeks/months previous.