Yeah, doesn't surprise me that harder water made better beer, no need to add lots of salts to it. My understanding is that soft water is only really good for pale lagers.
Hmmm, ok, surprised about the pH, as that is usually the big difference between dark beers and pale ones, the one that really jumps out. Are you using a different base malt between the 2 maybe? I ask, as first time I used Crisp Maris Otter, every beer I made with it was hazy, whilst at the same time beers I made with IREKS pale malt or Dingemans pilsen were nice and clear, same equipment etc, just different base malt... I switched to a different pale malt and fixed the problem, and got a better flavour in my beers (not saying it was because it was Crisp Maris Otter, could have just been that batch...).
The other thing I'd wonder about, well pales sometimes have more hops in them, especially dry hopping. Some hop varieties I have found can leave quite a bit of haze in beer when used in larger amounts. I don't cold crash or fine though (don't have the facilities to).