I suspect the reason you lost HF is due to the way your pad is arranged, the instrument input is constantly loaded with 1350 ohms, which is way too low for most instruments.
What's more, in the mic positions, the xfmr input sees too high an output impedance from the pad.
You have TWO Zobel networks on the xfmr's secondary...but I bet none is correct. Anyway, one is enough; must be adjusted under test (unless the xfmr's manufacturer has very precise info).
The input cap (C10-10uF) is not needed. If you connect directly the secondary to the non-inverting imput of the opamp, you'll have better performance. YoR18 is not needed; just adds noise.