The hump you are seeing at 6-8hz is called Hemholtz Resonator Response. I to have this hump in my living room that has a 44"x4ft deep entrance opening going to the kitchen. When I first got my UMIK I thought there was something wrong with it since this hump showed up in every graph I took regardless of position of subs or settings. Mine is more pronounced and centered at 10hz.
I emailed the guy in charge of calibrating the UMIKS at Cross spectrum. HE had me send him some graphs I took with REW. He then asked me If I could take one of the subs to a bedroom that could be sealed off by a door. So I lugged one of my Cap S2's into the room and set up my equipment ( that was alot of fun, haha). Ran some sweeps with the door shut and it was flat to 5hz. Opened the door and the hump came back at 7-8hz. He told me that this is classic Hemholtz resonator response. Basically our single opening to another room acts as a very large port in the simplest explanation he said. Where the hump is located in the frequency band and how big will be determined by the size of your opening and to some degree the depth of the hallway or length going in to the next room.
I dont have any EQ setup yet. Do have a balanced minidsp that I havent hooked up yet. When I do I dont even know if I would even tame my peak there, I look at it as just free power in the area. I know 10hz material really hammers in my room so I'll take it.
Here the only graph I have on my phone but gives you the idea of my room and my hemholtz peak.