The passive pickups in your ukuleles "hear" the sound of the strings through the bridge saddle. There is a small filament that sits under the saddle and listens to the vibrations. This signal is sent as a very low voltage signal to the output jack of your uke. If you play the "A" string, it vibrates 440 time per second, and the voltage frequency that the pickup creates is also 440 vibrations per second. This is sent out to whatever amplifier you use where the voltage is increased, shaped, and sculpted to taste and ultimately output to a speaker, which then vibrates at 440 times per second and we hear the note "A".
With a passive pickup (that usually has no onboard "tone shaping" tools such as Mid, Treble, Bass, or presence) you must rely on your amp to shape the sound to taste. An electric guitar amp is not at all the ideal tool for this. It is designed to work with a very different signal coming in the front end and will be limited in how good it can sound.
Something with a "full range" speaker (such as a PA speaker, or an acoustic amp, or even a bass or keyboard amp) will do a much better job of this. The other option is to use some sort of D.I. (direct injection) box between the uke and the amp input. An example of this would be and L.R. Baggs Para acoustic D.I. or L.R. Baggs Venue D.I., or the BBE Acoustimax pre amp pedal. This gives you tons of appropriate tone shaping tools.
Some pickups will sound better straight into a guitar amp than others, it depends on the impedance and frequency response of the pickup itself, as well as on the amp.
The best thing to do is just get in there and try it. The EQ (bass, treble, mid, presence) knobs will have the most effect, and you may have to turn the volume up much higher than you are used to when plugging a guitar into the amp.
Hope this helps a little!
