I should've said the amp model you want depends on the Bright switch, I think it needs to be a + model to emulate the V. In the V's manual it says they configured the Mk IV Mode for more midrange gain so it's different enough to the IIC+ mode. Which implies it's modelled after the IV in Mid Gain mode, and I think this is the same with the Extreme mode.
So I think for the IV mode you want USA Lead + or Brt +, with Fat, Bright and Presence shift on and for the Extreme mode you want the same but with Presence Shift off.
I think. Could be wrong. We're digressing...
Back to Quantum, I'll throw a clip together tonight with a real Mark V and the Axe configured to sound like the IV Mode on the V (because that's as close as I can get). The flubbiness doesn't feel right, I'm just not sure whether it's me or a bug in the beta.
The low end was the same on FW19. I just tested it again.