I just received MB Quart QP 805 M POS today. It's the version used in listening booths, 300ohm. It should be equivalent to GMP 8.300 D. I wonder how they compare to the 35ohm model discussed here.
According to the Ebay seller they should be new, so burn-in will probably change the sound significantly. So far I notice somewhat strong bass, a little overbearing, recessed mids and clear, but smooth highs. The soundstage is rather small, but okay for closed headphones, instrument separation is very good.
Isolation is worse than I expected. Comfort is not stellar - the cups are not deep enough so my earlobes are touching the driver covers, but overall it's not so bad if you consider that they are closed phones with high clamping force and pleather pads.
Btw, the pads' inside is round, not oval like the GMP 8.35 original pads shown here.
I hope the bass will get more definition and the midrange will be brought slightly more forward with burn-in.
PS: It has the steel-shielded cable, I wonder if I can get it off somehow, it seems permanently fastened to the cups. I mean the steel part.
As I listen more, the soundstage is actually pretty good - not very big, but positioning is precise and 3 dimensional.
UPDATE: Still very preliminary after ~2 hours of listening/burning-in. The highs are not as smooth as I thought and on some recordings can be sibilant, burn-in should improve this. The bass is still bloated a bit (the decay of bass notes is longer than it should be, other than that I would say that the headphones are plenty fast), but I dont't think it messes with midrange as I wrote earlier. The mids are not so recessed anymore.
Overall they sound balanced, neutral and natural (considering these are closed phones I would even say very natural) and are enjoyable.
I've tried them with all kinds of rock, some electronica - Rush, Prodigy, Tool, Perfect Circle, Dire Straits, Zappa, Pearl Jam, Radiohead, Flaming Lips etc. and they are ok with everything except DM, which sound really awkward - congested and muffled.
They are probably the best closed headphones I've heard, but that's not many - Senn HD280Pro, Denon AH-D1001, Beyerdynamic DT831 (these have worn pads so the comparison isn't fair).
I'm leaving for weekend, but will try to post further impressions next week.