The first question is, why do you want an instrument tuned lower than a soprano ukulele? The soprano, concert, and tenor are tuned the same,A-D-F-#B(the a is a fourth higher than the D, rather than a fifth lower, as you would expect) though lately, people seem to favor tuning a step lower, G-C-E-A. The Baritone is tuned to the same pitches as the highest four strings on a guitar D-G-B-E),unlike the others, the D string is the lowest, (though some people tune it high so as not to lose that effect)--

I tend to play the tenor most of the time, because it has a somewhat more mellow sound, and the neck has a bit more room to maneuver--The little ones cut through better when playing with other instruments though--

The banjo-uke, is basically a soprano ukulele with a banjo body, and then there is that thing that George Formby used to play, which had eight strings, tuned as a ukulele, about the size of a tenor, but the strings were metal--I would have to look it up, but I think it was a mando-uke--

No doubt that the banjo resonator is louder, but the sound is also much more harsh--the ukulele blends really well with the voice (it was popular with college singing types before the guitar was) and when you move to a banjo resonator instrument, it tends to overpower the voice-- Formby's instrument worked well for the music hall stage however--