If it sounds OK it is OK. The 700 RB II is a good amp but at this point I see no reason to get it over the Mb500 -- tone is just as good and it's 1/5th the weight and the same price or less. Don't worry about it until you've tested them out with an amp, since you've already got them
As far as replacing drivers, I wouldn't bother with those cabs. If they don't do it for you as is, put them on Craigslist and buy something better.