ok a couple of months later, now I had the 3 hours of luxury time to open the bass... and I used the cardboard-holding-the-pots to compare sounds and taper...
250k audio
500k audio
500k linear
all cts
with 2x Barto M4 quad coil with the double switching (serial-parallel and P-hb-revP) in Vol-Vol-Tone
first conclusion: 500k gives more top even at full (which does not make sense to my little electronic knowledge as the pot is shorted completely in that situation but there was an obvious difference)
second conclusion: the linear pot gives more musical taper (more smooth) than the exponential audio one, from full to 95% empty, then it is very steep. Again it does not make sense to my electronics limited knowledge (RC filters interact with the coil of the pickup to but I don't get it - I only get why the last 5% of the torque is very steep)
So I put linear taper to both volume and the tone. For the latter, it makes it even more useless than the audio taper, but for both volume I get much more subtle useable adjustments.
So I presume I'd like a linear for the 1st half, and exponential for the rest, but custom tapers are not really an option. I know that high end mixing desks do this so I presume this is where my bias comes from... my EMG active pickups with passive VVT is also so continuous and beautiful that I suspect there is a custom taper there too...
Anyway, I've lost enough money/time with this, and since I use only the tone to both extremes I will be fine ;-)