i had the same problem with the smaller 624 until i fixed the room
the advice from ethan and glenn ( and the rest of the guys ) was what fixed it for me - pure genius .
you have to be committed though , to improving the room - its pays off in spades with the big sound you get when finished.
loaaaaads of basstrapping and re positioning of the mix setup to face down the long wall , set up in the middle of the end wall
the window end is good if you put movable bass trapping into the window area ( i did this )
you can put monitors against the wall if the corners and wall are heavily bass trapped
i have up to a foot thick of bass trapping in the corners wall to ceiling .
now they sound amazing low end is incredible and my mixes sound the same on any system i play them on
i still smile every time i switch it on ;-)
see room pics here :
studio - a set on Flickr
ignore the chart , its way better than that now .
note due to the amount of bass trapping i had to set the shape switches to normal - and put the monitors on their side - but now the room sound nearly flat at the mix position plur or minus 6 db - best i can do for a cubic room