It is very difficult to say what the optimum pH is for a naturally-leavened dough. The pH value is dictated to a great extent by the fermentation temperature. If you were to use only a room temperature fermentation, it might be possible with your particular leavening system to state a range of pH values over time. But, if, as in your case, you use a combination of room temperature fermentation, cold fermentation and a temper at room temperature, the pH values over the same time period will most likely be higher, because of the introduction of the cold fermentation period. Also, where the dough will be from the standpoint of pH over the entire fermentation period will depend on the condition of the preferment (poolish in your case) at the outset. If it too acidic to begin with, then that will lower the final pH at the time of baking. That is why Prof. Calvel says in his book The Taste of Bread
to be sure to refresh the starter/preferment culture before using it (which, of course, you have been doing). If you have a pH meter, you might want to use it in your experiments and note the relevant values, particularly at the time the experimental doughs are to be used to make pizzas. If I had to venture a guess, a pH value of around 5 might be indicated at the time of baking. The range of pH values will, of course, be quite wide over the entire fermentation period. Since you did not experience any problems with oven spring from what you said, that would suggest that your dough pH perhaps was not out of line. It may have been more a lack of residual sugars to contribute adequately to crust coloration.
For background purposes, you might also want to read Reply 5 at http://www.pizzamaking.com/forum/index.php/topic,853.msg7771/topicseen.html#msg7771
, including the link to the theartisan.net website at http://www.theartisan.net/The_Artisan_Yeast_Treatise_Section_Two.htm
. The portion of the theartisan.net article that you want to read is the section entitled "Acidification". That section recites a fair amount of material from Prof. Calvel's book The Taste of Bread
. You will also note that the Acidification section talks about using malt extract (I believe that diastatic malt is intended) to reestablish the proper sugar balance. In his book, Prof. Calvel also suggests using sugar at 0.3-0.5% to restore that balance. As I noted earlier, I would like to see if you really need diastatic malt or sugar before adding them to your dough to get more residual sugars.
Another post where I discussed some of the above matters is Reply 13 at http://www.pizzamaking.com/forum/index.php/topic,8011.msg69043/topicseen.html#msg69043
When Tom Lehmann talks about sugar helping with crust color, I believe that he is thinking of a shortened fermentation period (most likely a cold fermentation period) during which sugars in the dough, both added and natural, have not been completely exhausted. As such, they would be available to contribute to crust coloration. This position would be consistent with Tom's discussion at the PMQTT at http://thinktank.pmq.com/viewtopic.php?f=6&t=4669&hilit=#p26890