I've been not using Vienna for very long time, so i can't remember where the 'fixed key/vel' parameter placed.
But if you look at the SF2.1 spec, they called as "Keynum" and "Velocity", they are parameter No. 46 & 47.
Both of them can be seen clearly within newer sf2 editor like Polyphone or Viena (one N).
I'm sorry, i use different names for them.
Yes, you are correct about the 'fixed key', that it's quite similar with the 'Scale tuning' and same result can be achieved.
And I also had explained that the 'fixed velocity' can also be achieved using the modulator function, but the modulator not yet supported by BASS.
IMHO, volume setting is essential in sf2 designing and sf2 spec only have the "Attenuation" where the volume can't be higher than '0 dB', so overriding volume level for samples at low velocity can be done by using 'fixed velocity' or the modulator.*I've checked also that make 'Attenuation' setting to e.g: -6, won't make the sound louder in VMS or XMPlay, which is a true condition.
So, if the modulator can be implemented, then i think the 'fixed velocity' can be ignored an use modulator instead.
Well, at least modulator for Note-on velocity (Key on velocity) to control parameters like Volume/attenuation, Filter cutoff or Sample-start offset... will be very nice.
And hope the 'keynum to hold/decay' can be included
For simple saying about the sound quality of BASS engine...it's Amazing!