There are so many variables here...
First of all, a Marshall electric amp for a uke, yuk. The solid state acoustic amps they do are pretty good though.
However, I wouldn't say that all acoustic amps are good for ukes, as I find the AER compact 60 not suitable for anything with nylon strings. They work fantastic with steel strings though. It just depends how they are voiced.
I actually do like my uke through some valve amps, but certainly not all.
The Epiphone Valve Jr would not be my pick for a uke. These amps are designed to break up early, so yes, at low volume it sort of works but for a gig I don't think it is the right thing. But, if you are a guitarist, get one, they are so much fun!
Personally, I found that a small PA sometimes delivers a better sound than an amp.
But we haven't even started talking about the pickup in question, as that is very important too. Those Mi-Sis have very high output, while the passive are often quite trebly with low output. So you might find that you need a pre-amp of some kind as suggested above.
So, basically, try before you buy! Listen, try out all variables, get the store person to set up various amps, PAs and also bring out some preamps to play around with. It's a bit of work, but unless you have an experienced store person that can guide you, you are better off trying out the different options. Don't rule anything out, you might be surprised by a basic transistor amp.
Anyway, that's my two bob.