Replace or Augment EF-S 15-85mm f/3.5-5.6?

[thekingb]

You might consider keeping your existing lenses and adding a fast prime for the shallow DOF and indoors shots. I'm very happy with the Sigma 30 f/1.4, which frames at just shy of 50mm on a FF body. It's a great workhorse lens that can be used in so many ways. Then you can keep the excellent 15-85 for outdoor use.

Edit: set your 15-85 at 30mm and walk around with it. Try out the focal length for a day. I find it to be very natural, which isn't a surprise given that 50mm on FF and the human eye are about as similar as you'll find in lenses.