mcn ad sales

Hi, I got an email saying that I was invited to join Maker Studios. I've heard a lot of bad things about them - like them not releasing your channel from their CMS, but I see that the bigger YouTubers are actually partnered with them. I always monetize my videos, although they don't make a lot...

Let's say your channel will have earned $1,000 in adsense revenue if you were not partnered. If the MCN also sells ads, let's say you earn $300 from the MCN. Did the $300 the MCN sold in anyway reduce what adsense sold or are MCN's only serving ads on non-monitized views.