@ Wazkelly One other thing you can try checking on your Ash J Bass is the neck relief, Fender specify a clearance gap of .012 of an inch between the top of the 8th fret and the bottom of the strings, you can use a standard set of Automotive feeler-gauges and a capo to check the neck-relief on your Bass guitar, the first thing that you do is place the capo on the neck just behind the first fret in front of the nut so that all the strings are resting on the first fret, next, select a .012 inch feeler-gauge, then fret the low E-String at the 17th fret, using the .012 inch feeler gauge, check the clearance at the 8th fret while you're fretting the string at the 17th fret, the .012 inch gauge should just fit between the 8th fret and the bottom of the string, tighten or loosen the truss rod till the .012 gauge does fit, then do the same for the G-String, I've found that the neck relief, and truss-rod tension also has an effect on the intonation adjustments too.
Hope that is of some help mate.