An ideal pre-amp should be completely transparent, to the point where you should not be able to tell the difference between it being on and off when everything is flat, assuming you have good leads and a reasonable amplifier.
What would be the advantage of the pre-amp having a 6dB gain, rather than, say, turning the volume up? It gives you a bass you can't switch to passive without changing your settings, or change basses live (like having a G&L!) I have never heard anyone complaining that the problem they had with the barts was because they are too quiet - the volume is fine on them, just that the pickups are a bit bland.
I think if the EHBs had nords, I would have got one by now.