No power available so no amps? Hmmm. Although sometimes it's just simpler to use a 1/4" cable, like when sitting in on somebody's rig for a jam...
As stated, unplugging the 1/4" cable from the bass is the only way to disconnect the battery (er, unplug the amp side first, duh).
Yes, if the 1/4" jack is stereo you'll need to sum the two channels to keep everything happy. If you have a cable with a stereo plug at one end and split into two mono plugs at the other, and the amp channel has two inputs, there's a good chance you could just plug in both cables to the amp and that would work. I believe there is usually a little summing resistor between two inputs of an amp... (like the bassman schematic shot below). Not true of a DI box's "in" and "through" jacks, those are directly linked.
Weirdly, the value of the summing resistors is not critical. There's probably a minimum, but it seems 20k is commonly mentioned - one for each output and then the mono sum is where they meet. If you're tricky, you can fit these into a right angle 1/4" jack and just use a mono cable. But that takes some finagling and a plug with the space.
Same great tone! Our preamps have the amazing ability to run off anything from +/-9 to +/-18vdc. But only a few hours out of a pair of 9v batteries. Now a couple of car batteries would keep things going for quite a while.
The stereo jack on the bass
will run headphones but results vary widely because impedance values aren't quite right for many headphone models. Plus you might need to crank the output trimpots... It's not what the bass was designed to do but can be a quick way to check if it is indeed a stereo jack.
Jimmy J