I second the 26-99. 10 loops is too much for proper flow with a 15-58, although it will likely still work if you're determined to go that route. I have seen quite a few 10-12 loop systems fed with a 15-58, but the delta T is pretty huge- like 40-50 degrees. Anything over 8 loops I go to a 26-99 to keep the delta T tight. At 10 loops I go to speed 2, and at 13 loops speed 3. Did one last fall with an 8, 10, and 11 loop manifold on 3 zones. Each got their own 26-99, with the little guy on speed 1 and the others on speed 2. They balanced out to ~.70 GPM per loop and settled to a 24 degree delta T. I also run my loops 250ft max. In your case you'd be maxing out speed 2