70% hydration is not super high. Tartine is around 75%, IIRC. But if your bread is "fairly dense" you might have other problems. You might find this bread troubleshooter useful. It shows pictures of various crumbs and what the probable cause of the problem is.
Most likely cause of a dense crumb is underproofing, though a too-cold oven won't help either.
The most likely cause of a tight (but not dense) crumb is knocking too much air out during shaping. You can try varying the ratio of bulk ferment time to proofing time and see if that helps as well.
> The most likely cause of a tight (but not dense) crumb is knocking too much air out during shaping.
I'm certain this is it. I worked as a baker for a few years, but I was making bagels. So my feel for dough is all based on that dough. Very different stuff.
I can't disagree with this enough. Gimme that hard, crispy crust with a good ear. And lots of good holes.
Although, my crumb always ends up even and fairly dense. No clue what to change to open it up. (Even with 70%+ hydration).