Duncan P-Rails problem in P-90 bridge sound

I have the Duncan P-Rails installed on my Music Man Axis, but am experiencing a problem with the P-90 bridge sound. It's too shrill and not loud enough, compared to the P-90 neck or even to the P-Rail bridge sound.
I installed a Freeway switch and asked my luthier to add a switch on the backplate of the guitar. I can access all 4 sounds (humbucker, P-Rail, P-90 and parallel) this way. My luthier says it's "normal" because it's a hybrid pickup and there's a tremolo on the guitar, but I don't buy it. I wonder if it's wrongly wired (although he's a very good luthier) or if there is maybe a fault on the bridge pickup.

Does anyone have the same problem? How does your bridge P-Rails P-90 sound compared to the neck pickup?

Quote from waraba

Yeah cool pickups , I've got a couple of these on my Yamaha SG1000.

Quote from waraba

I don't know if you can measure voltage or impedance with a multimeter on both coils , but this way you'll be able to see if both pickups have the same output and discriminate this potential problem?

I actually did that yesterday night, after sending my post, and the resistance is OK - matches the documented values.

Quote from MuddySludge

Switch to the single coil first, then to humbucker. It probably won't get raspy until you switch to the P90.

That's right. That's what I am experiencing too. I will just make a special rig just for the P-Rails P90 then on the Kemper. I will look at the Input options too like you suggested. Cheers!