My gut feeling is that you have done plenty well enough with a 690. PhD program stats are not figured in for MBA rankings, so they will not be as critical of your GMAT score from the angle of improving their ranking by only admitting 750+ students. They want to know you are smart and that score certainly helps reassure them. The reason so many folks worry about a really high GMAT is because the top schools want their average to climb every year, but again, the PhD programs are not counted in with these stats (neither are the part time or executive scores by the way). Hope this helps. Great job on the GMAT and good luck furthering your education.