Help mixing two active Signals

[garfo]

So, here we go.
I have been on a quest to mix the two pickups on my bass. Both go through a preamp each and both have a 10k Volume pot at the end. I have designed a simple Summing amplifier to mix them in two ways.
The first time I used a Panning configuration and left the Volume pots out. Because I couldn't find the right type and value of pot(needed 10k center detent with long bush, and could only find a 50k), used the 50k and when in the center position there was too much volume drop. Tried with a 10k and it was great, but I just can't find the right type to use it as an onboard potentiomer. So I tried using 2 Volumes coming from the preamps and mixing them with this circuit. Now the effect is the opposite, it sounds considerably louder when both pickups are full.
Is there a way of using this 2 Volumes configuration where in the middle position the output will be simillar to when only one Volume is on Full?

Thank you in advance.
The Schematic is attached