You upgraded from a 550D to a 70D and complained in another thread that it gave you blurry pictures. Others requested you post samples in that thread. What ISO was used was one question that could have been addressed. Instead, you returned the 70D and got a 6D and now complain it is too dark, even though the 70D was good. All this in just a couple days. Thinking you bought 2 bad cameras in just 2 days. Hmmm....
Personal preferences do influence how we like our pictures to look. Our minds are still better than the camera in difficult situations, I adjust exposure to compensate when the camera has a different thought than I do for "how" I want a particular scene to look. You need to not look at your cameras being "faulty", and slow down to get lots of experience with your present camera body. You have excellent equipment to shoot with! Even the 70D. It is not a camera issue you continue to have, but one of simply experience. Posting the photos with the EXIF File will help you tremendously. Lots of guidance is available with some very experienced people here. Please share your pics, and they will be there for you.
Purchase the book "Understanding Exposure" by Scott Kelby. Study it thoroughly and you will come to a better understanding of the part you play in making good pictures. "Cameras don't create good pictures, photographers do" is a trustworthy old saying. Your system is fine, just be patient in learning it. By the way, I generally shoot both my 40D's at +1/3 EV.