try the Lace Drop and Gains. They are made for drop tunings. They articulate as you drop your tuning down without any degregation in your note (s). Added bonus; passive no batteries needed.
I had a set of these in my baritone that I tuned down to A and it sounded incredible.
Unfortunately these people are looking for 7 string pickups and Lace only makes 6 string Drop and Gains.