Might be that the amp has some kind of preset tonal colouring which involves scooping the mids/upper mids (say a narrow band of frequencies anywhere between 600 and 1200Hz). SWR started it off and called it the "LA sound", then Trace and Eden followed with their own versions. Jazz basses sound great and so do basses which sound brittle. But other basses (especially with aggressive mids or treble) just sucked. Warwick did it too but less in favour of jazz basses and more for their own. Their amps sound great with Stingrays BTW.
Try a GB Shuttle instead. Pure as the driven snow and you can stick a mid scoop in if you like.