Help?

I have a split level home, 1 bath upstairs and a bathroom that was added to the downstairs area some years before I bought the home. I moved in, had the bathroom redone and now, when I use water upstairs (shower, dishwasher, faucet) the downstairs tub backs up with brown water that smells dirty, kinda like rust, grease and whatever else. Had the downstairs bathroom snaked twice, helped for a minute but didn't last longer than a month. Camera went down and said there were no branches or roots in my sewer line that the could see. Had three plumbers come out,and no one can really give me a clue on what to do. One plumber mentioned something about Venting, but it was more like, "maybe that is what it is but Im not sure" Another one said I may need to just tear down the entire downstairs area where all the plumbing is and redo it all.... any suggestions? im frustrated and about ready to throw in the towel ....

Original article and pictures take http://www.hometalk.com/118685/help-i-have-a-split-level-home-1-bath-upstairs-and-a-bathroom-that-was-added-to site