The Warm and QTUBE voicings of the V803 are both based on the tone of a classic, vintage tube bass amp. Upon first play, you may suspect that these two voicings are actually identical. But this isn't completely the case. Here's the difference between the two:
Warm - Tube-like output character with bottomless limiter.
QTUBE - Tube-like output character with ordinary clipping at high gains.
Essentially, both voices are dynamically different from each other. The Warm's bottomless limiter will help prevent the low end from "bottoming out" and the top end from choking/gaging (similar to the behavior of the Modern voicing). Whereas, on the QTUBE voicing, the limiter is instead replaced with a clipping circuit, so you will be able to achieve more breakup with your Gain up adequately.