OP,
Download a owner's manual for the GK MB-500 heads.
In there you'll find it says, "For 800RB voicing run the contour fully on or fully off."
I gigged with a 800RB from 1984 to roughfully 2008.
I now gig with either a MB Fusion 800 or a MB500.
It has been awhile since I heard/gigged with the 800RB, but I believe with the MB's contour fully on it's, "not exact", but in the ballpark.
I always have a SansAmp BDDI V2 infront of the MB500 head, and to my ears anyway, seems to bring it even alittle closer to the 800RB.
And a bonus, if you watch the T.B. Classifieds, used but not abused, MB2-500's and MB500's pop up often around $250.00 to $300.00.