There isn't really the best version. I believe the A is more closer sounding to a IIC+ with less gain in R2 being forced to push it more while the B has more of a modern sound to it with more gain in R2 but these are super small differences, they are almost the same I think.
http://forum.grailtone.com/viewtopic.php?t=5987
That might help you a bit. Don't start thinking that one is better than the other they are both awesome versions. I own a MIVb btw. Get one and you will never think about getting another amp ever, it's just amazing.
