Mag pickups breaking up at full volume

I’ve got a Majesty 6 which I’ve owned since new in 2015. It’s always had a higher output compared to my other guitars but I’ve noticed that lately when I strum harder when using the mags, the sound is breaking up and it’s very noticeable on clean sounds. I thought it was the batteries but these have been changed a few times. I also replaced the preamp and all switches after a battery drain issue but the sound problem with the mags is still there.
The only way to resolve it is to roll the volume back a little. It’s almost like the dB boost is on (it’s not – if I activate that then the problem is multiplied).
I can’t lower the pups as they are direct mounted.

Any thoughts on the issue here? It feels like the pre-amp is set too high. Is there a way to tweak it?

Re amp, I am running a Y cable into a Helix – the mags straight into the guitar input, the piezo through the FX return.

Thanks!