Any suggestions for a replacement preamp for my Fender Zone bass. Its direct wired now with volume and tone control only. Any help would be much appreciated..Thanks

See FAQ the bass mod link in the first paragraph of Dimento's Trouble Shooting Guide.

Don't know the bass but apparently it has a single pup which means obvious choices would be (given two potholes) a single stacked active two band (v/b/t)or double stacked active 3 band (v/m/b/t) or double stack two band active/passive vol/tone b/t - if you drill for a switch).