This may become my favorite bass app on iOS. There are lots of ways to generate synth and fret instrument bass sounds on iOS, including synths, samples, and SoundFonts. But iFretless Bass is a great pick-up-and-go package that has real life quality and touch responsiveness. Its AB and IAA compatible, too.
The strings can be laid out in various intervals and there are about 10 instruments, including some decent synth sounds. While I found the string interface easy enough to play, it should be noted that the app works as both a MIDI controller for other apps, AND you can use other Virtual MIDI apps to control it. So if you have a good MIDI sequencer (I recommend Xynthesizer or Analog MIDISequencer, but there are many), you can generate perfectly timed rhythms that are precise even beyond what a human could actually play. This is particularly great if you just want the bass part as part of a backing track for other instruments to layer on top of, such as a synth or guitar.
The EQ controls actually give a decent variety of sound control under the hood, but I think the variety of instruments here is pretty good to start with. Rock/guitar musicians often feel a little neglected by the bass and drum options on iOS, which tend to be EDM-oriented, but not so with iFretless Bass. While the app did seem to run well enough on my iPad2, there was some occasional clicking during faster passages that is a bit of an annoyance. I believe the developer has a YouTube video about how to reduce/circumvent this, but this is a minor occurrence that should not discourage you from checking out the app. Highly recommend.