You'll probably find it easier to read AC voltage than current at the output, but yes, you should see a reduction as you lower the trim pots. Hit a note, let it sustain and watch the multimeter and you'll get a pretty good measurement of the output.
The steady state level that you need to drive the preamp with is suprisingly low. Alembic doesn't show the spec online, but it's typically on the order of 50-100 millivolts (so .05-.10 VAC) at input 1. Input 2 is the same with a pad to drop the sensitivity by 6dB, so that must be something like 200-400 millivolts to drive to full level.
With a regular (digital or analog) multimeter it will read average voltage levels (which is what you care about). You need to have fancier equipment to capture the transient attack when you pick. With an oscilloscope, you might be surprised to see a 1500 mv attack on a note that ends up sustaining at 50mv!
I don't know what the range on a Signature bass is, but a Series bass can drive many power amps to full power, which is something like 1500mv (1.5 VAC). The Series electronics are run on a higher voltage than Signature, but I doubt that there's much difference in output level.
For the F1X, the tone should be the same with either input, it's just that #2 cuts the input signal down to prevent distortion. So, what you should do is set the bass up the way you like it, and use input #2 if it's distorted sounding. The tone really is the same between the jacks, but if you compare the two outputs without changing knobs, input #1 will sound so much louder than input #2 will sound wimpy (your ear will normally read louder as better unless it's distorted). Some other preamps may have different tone in the different inputs (Ampeg has bass cut on some inputs of the SVT, and some amps present different impedances at the different inputs which will also change the tone).
If you switch between instruments, then setting the output level on your Alembic low will make switching easier. Again, because of the active electronics, lower levels don't compromise tone, but your ear will trick you into thinking that you're making the bass sound wimpy if you listen while changing levels. If you don't switch, or don't mind tweaking knobs when you do, you can up the output level of your Alembic and get a little better signal-to-noise ratio (and make all your other basses sound wimpy by that same psychoacoustic effect).
My guestimates on sensitivity levels may be off here and the factory folks can correct them, but these would be pretty typical values for most amps.
David Fung
