Bartolini preamp for TRBII-6?

I have a Yamaha TRBII-6 I picked up cheap from GC recently- they gave me great price on it because there "may" be something wrong with the pickups/electronics. It sounds fine at low volumes but tends to sound harsh at high volumes, I pretty much have to back the treble completely off since it's so bright and the mid control sounds really weird to me. I'm wondering if swapping out the preamp would help- the Bartolini looks like the closest fit, but I'm kind of new to modding electronics so I'm looking for advice from anyone who may have changed the pickups or preamp in one of these basses. I'm not sure if I need to replace the pickups, but the preamp would seem to cost a lot less. Any advice would be greatly appreciated- thanks.