Sanderxpander wrote:This should be fairly easy to test definitively, by playing a single note and recording both midi and audio, then playing back the midi and recording audio again.
 Have done a bit of that reverse engineering.
Testsignal: A dry Steinway B Prelude with Limiter off. (a 1kHz sine would have been easier, but not available inside Pianoteq)
Recorded two notes c3 with different fixed velocities 127 and 64 inside Pianoteq . With audacity I measured the peak-levels of the wavs. For each stereo channel separately:
At velocity 127:
L: -6.8 dB  R: -9.9 dB
At velocity  64:
L: -18.6 dB R: -22.1 dB
Delta:
L: 11.8 dB  R: 12.2 dB
Result_1:  At half velocity 64 the output level is ~12 dB less.
Pianoteq's VU meter doesn't have a dB-scale, but I found that this 12 dB volume drop was equivalent to 4 marks on the green bar. 
Result_2: One grid mark on the VU meter is probably equivalent to 3 dB.