For what it's worth, belated notes on my 504:
- I saw a few online reports of problems with the passive/active switch, but I've had none: it does what it's supposed to every time, and I can flip the switch while my amp volume is all the way up and there's no "pop".
- The preamp has a little hiss. You probably won't hear it in most situations unless you've got the treble boosted. It goes away in passive mode.
- It seems well-shielded. In situations where my other bass picked up some annoying hum, this one doesn't have any.
- Out of the box the action was low, with no buzz, all it needed was tuning. Since then I've changed the strings and had some trouble getting it set up as well again--but I'm pretty positive that's me, since I've never attempted relief/action adjustments before. I'll give up and take it to a pro some day.
Anyway, executive summary: great instrument, I'm happy with it.