My abs definately became bigger, chunkier and more visible when I started working them hard 3 times a week with 8-15 reps (roman chair situps w/ plate on head, weighted leg raises etc...).
Things like squats, deads, and standing shoulder presses will also hit your abs hard too, and give them that chunky lookas well.
at 13% bf, you "should" be able to see a decent bit of ab....at least I could. They didn't get rock hard though, until I went sub 10%.
If your doing squats, deads, presses, etc...Don't worry too much about your abs...just get your body fat down.
btw...how long have you been lifting? 5' 9" 250 lbs. 13% bf is pretty crazy;)