I'll give you my take:
I just bought 2 avatar hellatone 60L speakers (which are essentially marshall V30s that have more bass response or something). I just reamped for the first time and I can't tell you how much of a difference it made. Playing it, I could see a little bit of a difference, but there are still two G75s that are in there, in the X config. But once I put a 57 up to the amp, I was blown away. I instantly found a much more pleasing and workable tone. I say do it. I don't know what the valveking cabs are made out of, but I find it hard not to be an improvement over what you have now... maybe if you want to save up for a bit, buy an avatar cab with V30s already in it. I think you can get a new one for 500 ish... and it's supposedly a lot like a mesa cab tone, but I don't know how true that is. haha