KZ6 fretless pickup upgrade.

I recently intstalled a Bart Ntmb in my MTD fretless KZ6. It wasn't a huge improvement but it was better than the original. I'm looking at swapping out the pups. I thought about Barts because I'm trying to get more warmth out of it. Do you know if Barts in the P4 configuration are direct replacements? The other option is Fat Stacks.
Any input is appreciated.