You are right, I have a problem with my measurements in the bass region and I don't know why. I used a Tin T2 for reference, perfect seal, and also get the bass roll off.
I've tried ARTA and REW, both with calibrated soundcard and microphone to rule out a software problem.
I've checked all windows settings for enhancements and stuff and everything is fine.
Right now I have no idea what the problem might be, other than that my 711 could be faulty.
--edit-- Sometimes it's the most obvious things that fly under your radar. It was my usb soundcard which somehow didn't want to measure low frequencies correctly. I bought it specifically for measuring but it was a cheap one so there's that. ^^