You can also cheat with citric acid.longer proof (fermentation), I do the quasi-no kneed overnight proof 12 hours in the fridge and then the usual regular proof 2 hours prior to baking.
Similarly, you could increase the amount of SD starter in your loaf.
My standard recipe is 3 cups whole wheat, 3 cups white, 3 cups sour dough, and 2.25 cups of water + salt but I usually err on the side of more sourdough.
After you've been feeding your sourdough for a while, you will notice clear "pot liquor" on the top in between feedings. This is caused by the yeast getting hungry and turning SD starches into alcohol.
I'm not a big fan of it and usually pour mine off but if you want your SD more sour, just stir it in before feeding.
#PS.TakeItToTheFoodMonkey...
Extra-Tangy Sourdough Bread
A chewy, light-textured loaf with a creamy, hole-riddled interior.
www.kingarthurbaking.com