Can't seem to make a light loaf, crumb usually comes out dense compare to store bought

I don't know how they do it, how does store bought bread get that nice light fluffy airy texture? And I'm referring to all bread, whether artisan, or factory made, they all have a nice crumb with light texture when you slice it.  Instead mine is always heavy and chewy even though there is plenty of air bubbles between the crumb.  

So I recently tried a sourdough conversion of cinnamon raisin bread recipe from Peter Reinhart A Bread Bakers Apprentice.  And it came out kind of dense.  It is expected because of all the addition of raisin and walnut, but I'm talking about the crumb itself.  I guarantee if I made it without the additions, it would also be the same texture.  The recipe says it makes two loaves but I was able to fit the entire dough in a single standard 9 x 5" loaf pan. Plus I kind of wanted a higher loaf compared to the one pictured in the book.  I was expecting it to be higher but it didn't rise much in the oven.   

I'm using hard unbleached organic flour (Canadian Manitoba which I think is 13 or 14% protein) and everything else was as in the book except I used 2% milk instead of butter milk.

• I did the levain over night (I think it was around 20% of the recipe total of flour and water)
• then I mixed everything in the stand mixer properly until it was just sticky at the bottom, texture was good
• I bulk fermented for about 4 hours, it rose a lot in the bowl
• then I shaped and placed in the loaf pan (9" x 5") and proofed for about 2 hours, it reached above the lip of the pan
• I baked it at 350F until it reached 190F, it didn't rise much in the oven considering I placed both loaves worth into a single pan

The only thing I can think of would be to 

- proof the loaf for longer before baking
- spritz top of loaf with water before baking and use pan with water to create steam in the oven
- looking at the recipes for the breads I buy many have additional wheat gluten as an ingredient. Could this be the missing key to getting that texture?  Or should I be reducing the amount of protein by using all purpose flour?  

What do you guys think? 
Should I try all of the above or just start with increasing the proof time?  I'm just afraid of over proofing.  

EDIT: Recipe is
3 1/2 cups (16 ounces) - bread flour
4 teaspoons - granulated sugar
1 1/4 teaspoon - salt
1 1/4 teaspoon - ground cinnamon
1 - egg
2 tablespoons - shortening (replaced with butter)
1/2 cup (4 ounce) - buttermilk (replaced with 2% milk)
3/4 cup (6 ounce) - water (replaced with 2% milk)
1 1/2 cup - raisins
1 cup - chopped walnuts

Sourdough modification is 
remove 20% of the flour + water to create the levain
so total is 16 ounce flour + 10 ounce water = 26 ounces
20% of that is about 5.2 ounce, that is how much levain I used to ferment overnight at room temperature
From the 5.2 ounce I split it into about 1 ounce starter + 2.6 ounce flour + 1.6 ounce water

I made sure to remove the appropriate amounts of flour and water used in the levain from the original recipe.
I bulk fermented for 4 hours and proof for 2 hours before baking.
I bake for 20 min at 350 F then rotate pan and bake for another 20 min or until internal temp of the bread reached 190F
It measured around 195 F when I took it out of the oven.