Please pardon my dumb question, but I want to clarify. If I have a uke with a passive pick-up, the iRig becomes the pre-amp? I believe that's correct. Definitely worth the $100 to have a portable pick-up for ukuleles without one, and to serve as a pre-amp for those with a passive pick-up.
No question is dumb and I am glad you asked this one in particular.
You can either:
a) use the MEMs mic that comes with the iRig Acoustic Stage system alone WITHOUT the pickup already in your uke
or
b) use BOTH the MEMs mic AND your passive pickup together, and your passive pickup goes into the AUX IN jack on the preamp, and you use the blend control on the preamp to balance the volume betw the two
or
c) use the passive pickup from your uke, and an adapter cable to convert 1 end of the cord from your uke down to 2.5mm so it fits into the MEMs mic input plug ALONE without, the MEMs mic, and this will actually give a very nice tone much improved over whatever you have now from the passive pickup with no preamp.
I have personally tested all of the above and know this to be true based upon my own experience.
All of the above makes a nice sound because use of the iRig Acoustic Stage preamp remedies the 'impedance mismatch' problem as discussed in the pages found from the link in my signature below regarding pickups.
It is science fact that ALL passive piezo pickups going into any system
without a preamp or DI box will have impedance mismatch and will sound thin, tinny and what is often labeled the 'dreaded piezo quack'.
All is explained if you follow the link in my signature. I've written in great detail about this many times before.
Hope this helps.