Honestly having read many comments about this amp and watched all those useless youtube clips i'd like to throw my two cents worth.
Yes, sure this amp will sound great with the mid selector set at any number, ditto the midrange and treble, playing solo or doing slapping party trips for your mates, but when the drums and guitar kick in, in a live situation if you DONT engage the Ultra Low, this amp is pretty much useless.
In bars and clubs ( without p.a. support ) playing anything from the Black Keys, Maroon 5 to Chic, engaging the Ultra Low - the audience will actually hear what you are playing, you definetly wont be lost in the mix.
So if you want a thumpy, ballsy ( non boom ) gindy versatile sound that will sit great on stage, just on top of the drums and guitar ( where the bass should be to hold everything together ) and you want to sound like a bass player out the front, try the following nominal settings. ( will vary between venues of course )
Note - Finger playing using Fender passive jazz 4 string - with an Ampeg svt 6x10 cab.
Ultra Low - in, Ultra high - out
Gain ( input ) - 2 o'clock
Bass - 12 o'clock ( varies )
Mid selector - position # 2
Mid - 2 to 4 o'clock, Treble - 1 to 3 o'clock
Compressor - if you must, nothing past 9 o'clock.