Razman, I'm assuming the bass came with reasonably fresh rounds? That would be a good start - if the strings need replacing. With the Selects and the passive tone controls, the only strings that actually sound bright are good ol' stainless rounds.
If you want to go the mod route in addition, you could also try the Invalid Link Removed. They are tremendously clear. If you pair it with the Invalid Link Removed, you can crank in as much bottom or top in as needed. It's much more the traditional Steinberger active tone, which may be more brightness than you're looking for. But it's a very different, more modern voice for a Spirit, if that's what you seek. I have an XZ-2 with that setup - installed myself, was easy - and can definitely recommend it. Probably somewhat less money than going the Bartolini route.
(I'm amazed that EMG's prices have not gone up more than they have the last couple of years!)
Or - you could grab an outboard preamp to boost output and the highs.