I have seen a few discussions about Sibelius Live Playback Velocities on these forums, usually relating to getting more realistic accent/tap dynamics out of battery instruments. Live Playback Velocities are also useful for making certain notes louder or softer than they would otherwise play back without futzing with hidden dynamics and other tricks.

My question is this: are Live Playback Velocities applied relative to the marked dynamic, or are they independent of dynamics? If the latter is the case, then the trick of changing all accents to a specific Live Playback Velocity will mess with the written dynamics, won't it?

The Live Playback velocities are independent of the dynamic markings. And yes, you would be correct in assuming that LPV would affect written dynamics. I find that if I decide I want more contrast, I normally just use LPV on taps and let the accents fall in the programmed range for the written dynamics.

It definitely takes a little massaging to et it dialed in just right, but that's mainly due to the fact that the lateral density and style of music (i.e. most music doesn't have such stark contrasts in dynamics) make it very difficult to program the software to respond accordingly. It's actualyl better now than with VDL2 since Jim tweaked the velocity ranges for 2.5.