I have a 6 string Grind and have also considered upgrading the pickups. However, the tone it produces currently is very even and I think considering it's passive, the tonal variety is not as limited as some would say.
Hand position plays a massive part. I use the bass for jazz, soul, gospel, pop and reggae mainly and it's covered every genre effectively and authentically. The strings I've used so far are Rotosound PSD piano strings (nice) DR High Beams (lot's better) R.Cocco Steels medium gauge (awesome but very pricey!) and also D'Addario Nickel rounds light top medium bottoms (sweet when new and lovely mature played in feel after a few gigs and a very reasonable price from Stringbusters).
I don't agree with the loudness issue, it's a double soapbar which must be louder than any passive P/Jazz.
I had my bass looked over by a shop technician and made my own bridge/saddle height adjustments. To say "the tone is in the hands" would be a tad blase, but there would be a merit of truth in that. I compared a Grind 6 with a 2 grand signature cirrus at PMT in Birmingham and the sub £400 Grind felt and sounded phatter than the Cirrus. I think the Grind is the best bass in it's price range and already easily competes with basses in the sub
£1000 mark. My particular model is roughly 5/6 years old and has the oil look finish. It looks great in photos, the Imbuya wings have a lovely grain. Unfortunately not every Grind looks as good, I've seen some with different coloured wings. So they can be hit and miss aesthetically but tonally I've had it sound deeper and clearer than a £1800 Sadowsky. I'm looking at Bartolinis or modified MM pickups. I'll report the results and if anyone's interested in what I sound like, check this link:
http://www.youtube.com/watch?v=-m900YmvnXQ&list=PL1A1AEE5968004BAE&index=2&feature=plpp_video
Bit of an essay, but this is my bass - I get great results from it and have never had complaints about the tone.
Active electronics with always have a degree of noise, passive is quiet if properly shielded and of course no battery to worry about.