Caustic has always had a processing quality better than nearly anything else on mobile. And now that Caustic 3.2 is out with its new wave editor, I guess it can really do some great stuff with vocals.
While it certainly cannot double vocals or tune them (it can, manually; but extremely inaccurately), it's possible to apply high-quality Caustic vocals, AND do a bit of other useful stuff, like manual de-essing by applying filters on sibilant areas. These vocals can then be played with the Beatbox machine.
Any other ideas about how Caustic can finally do something about vocals? Because nearly every other application I've used, has ruined the audio quality. Audio Evolution has some high-quality effects and tools, but the ones that won't screw the quality are from Tone Boosters; which you have to pay for separately.