Hamelman's Vermont Sourdough with Increased Whole Wheat (photos and a question)

I made the recipe almost exactly, with one exception: I used 20% WW instead of 10. It came out pretty tasty, just a wee bit sour which is what I like. Crumb:

Now for the question. I did increase the percentage of whole wheat (and accordingly, increased hydration just a touch, maybe 2-3%). Yet, 20% WW is pretty small, and I was hoping for a much more open, holey crumb - no such luck. Is increasing the hydration for this Hamelman recipe to get a more open crumb a good idea (assuming that's the solution)? Or is there some other method I should try to get it more open? Or could it be that my starter doesn't have enough mojo? I suppose that's a pretty open ended hard question to answer, but anyway, I thought I would ask. Loaf had fairly decent volume, though.

Thanks.