Hi Djones,
My D6000's have just come back from Meridian UK after being upgraded to DSP 96/24 - so I can enjoy the benefits of upscaling etc. They didn't come with either a certificate or 96/24 sticker (and that's the bit that's really important

) but they did have a nice high "electronics" serial number on the back. They were a complete back panel change (including the lower power supply part).
My room isn't as large as yours but does have an open extension - indeed one speaker is phyically in that part of the room. I've never played that high for any length of time - it's loud.
I wonder if someone's tweaked the bass or treble levels, or the boundary compensation and that's tipping the balance.
I can't stress enough the need to at least try the calibration - playing at 87, calibration level, I think you should get close to 75dB at say 10feet from a single speaker (start with the one furthest away from the open wall). If necessary get closer to the speaker to see if you can get this level out of it. If not you'e got a speaker problem.
If you do then it's still possibly a set-up problem or your room really is sucking up all the power. I had a sub which calibrated perfectly but wouldn't play until I found it's little red button (which doesn't exist on the 6ks).
Having tried 7k's in my room I don't think they'd make a big difference in level. The 6k sound was fuller and deeper to my ears and I wasn't aware of running them any lower in level setting. Of course their temperature sensitivity might be less than the 6k's so maybe that's a factor we can't judge.
Meridian equipment is complicated - at first it seems almost impossibly so - but once you get it going the rewards are great. Don't give up too soon.
regards
Chris