mostly at spot A there is a pin or protrusion of the metal to prevent the rod to slide out of the bracket. Remove that pin or protrusion by grinding or whatever. This way you'll be able to take the rod out of the bracket a little more and make the stand take a 15 incher. I did the exact same thing with the stand for the 15 inch floortom in this drumset:
http://vintagedrumforum.com/showthread.php?t=52163
If that is not sufficient you can make the rod longer by welding a piece of metal on it and make it flush with the rod.
You could shorten B if the stand is to high.
Quite a few vintage stands take 15 inchers though, first try.
Goodluck!
Maybe some other members have better ideas?