Question though: you keep adding to the dough without reducing for the first few days? The starter you keep in the fridge is of substantial size iow? Does this make a difference to the acidity level? (Mine is currently kept at 50+100+100g when feeding, so 1-2-2 of starter, fresh flour and water. It was started with rye but gradually and increasingly now more white wheat and some whole grain wheat or spelt.)
Yep, I try not to waste anything, so for the first four days, I just keep adding stuff. The starter is then kept fairly large. As far as I can tell, it had no perceivable level on acidity level.
From what I had read over the past few years, it's mostly the ratio of flour and water that affects the acidity level in an existing starter. More water than flour would make it more acidic, more flour than water nuttier/earthier and less acidic.
Prior to that (i.e. when starting the starter), it would be the flour used and how lucky you get with yeast strains. Also any additives used, if any (fruits, veg or vinegar, for example).
I have only experimented with started of mixed flours once and wasn't very happy with the results. It's like the rye yeast I caught didn't like wheat whatsoever.
Second, this means that the starter is a significant share of the final dough for your bread. I’ve used a much larger share of fresh flour in the dough, with starter at maybe 20-25% for the last bread I made. What difference does this make?
Digestibility (more carbohydrates and gluten broken down) and more flavour/stronger taster.
The former, as far as I can tell, means lesser rise of the loaf. This combined with me using only wholemeal flour, it also means very even bubbles, although usually not particularly big.
The latter means that more of the character of the starter is imparted, assuming the same fermentation time as with a smaller amount of starter. Whether this is desirable would depend mostly on the starter or what you'd like to have the bread with.
Both above mentioned effects mean the loaf is stronger tasting per slice.
I like it that way. It pairs really nicely with even stronger tasting cheeses than normal bread, resulting in a more balanced flavour.
On the other hand, when combined with gentler tasting toppings, it makes for a richer taste that I find more satisfying as it also increses ingredient contrast.
Oh, and it's lovely with a good stew or popped into soup to thicken it.
I also tinkered with non-grain flours, but they're tricky (and I haven't made sourdough using those):
http://www.thefreshloaf.com/node/43652/chickpeabuckwheat-pie