Your Series has active electronics, so there's no change in tone when you reduce the signal level with a potentiometer. This would be true for a master volume onboard your bass if it's after the preamps/EQ, and an external volume pedal or pot-in-a box as well. There's sometimes a psycho-acoustic effect that turning the volume changes the tone, but for active circuits it's not a measurable change to your tone.
There's one other thing you might think about. I certainly understand not wanting to cut another hole in the top! A Series II has a master volume which is implemented as a ganged dual pot. This lets the master volume adjust both pickups and still maintain true stereo output.
If you don't run your pickups to separate amps, you can combine the pickup outputs inside the bass and control the volume with a regular single pot. If you can find an appropriate stacked pot with individual controls, then you could move the bass and treble volume knobs onto a stacked pot and put the master volume in the freed-up hole.
Another possibility is to switch to a master volume/pan setup. I don't like pan pots myself, but this is a reasonable solution for some. The pan pot is a harder component to find.
Good luck!
David Fung
