Hi. I recorded a simple MIDI bass line through Scarbee PreBass on a MIDI keyboard. Generally, it sounds pretty good in terms of consistent volume levels from note to note -- except, for one here and there.
 
I went into PRV and was able to raise the Velocity of those notes, but even at the max (127), the notes aren't as loud as other notes, which are even at a lower velocity. I don't understand that. Is that something I could address with compression leveling once I bounce the MIDI clip to audio? Is that a good approach?
 
Can one of our generous MIDI gurus provide some guidance? Thank you! 
Billy

Velocity translates as the strength of the note being hit/plucked/whatever.  Normally this will mean 'a bit louder' as well, but not always.  
Think of hitting a snare drum:  tap it softly and the note is quiet, hit it hard and it's loud.  As well, the dynamics of the sound change.
 
I'm surprised if it's happening in something like Scarbee PreBass - i don't have it, but would assume that notes with a similar velocity would have a similar volume (unless it was changed in the track, but that would likely affect other notes).
 
Compression may help, or you could use a Clip Envelope in the track to raise the volume in the quiet notes.

Velocity is a numerical value with no fixed relationship whatever with any particular "volume" that it will produce. You can not even say for certain that the power of the output will increase with higher numbers, since the velocity value can be set to control other parameters of an instrument than volume, and conceivably an instrument can be set to produce a lower output to a higher velocity. That latter setting might be useful for example if you are using a controller to send to two instruments and you want the output of one to duck or drop as the output to the other increases.
 
If you want to control the output at the highest level the velocity value will control, you can usually do it in settings for the instrument itself. The same can often be said of the relative relationship of different velocity values via settings like a "velocity curve." There is no reason to assume that an instrument's output will invariably follow a linear relationship to the velocity number. Absent settings in the instrument to control its response to velocity, you will need to increase the gain in the recorded audio track or playback system. 

It happens sometimes that certain notes aren't as loud as others, but it ain't supposed to work that way. Library developers carefully level-match samples to avoid that, so it sounds like somebody goofed when the library was made. You might want to let them know about it.
 
In the meantime, you could try to mitigate it with volume automation. That'll work if the part's not overly busy. I'd do it with MIDI CC7 automation, so that once compensated you won't have to worry about it again when it comes time to automate the volume on the audio track.
 
Of course, if you're handy with Kontakt you could fix the problem yourself. But that entails per-sample volume adjustment, so I wouldn't recommend it unless you're already comfortable fiddling with Kontakt sample mapping. Even if you're not, you can safely experiment with it - just copy the .nki and work on the copy rather than the original.

Midi Volume and Velocity are different in some ways. Volume is obvious. Velocity depends on the player. 
Normally velocity is louder but it's intent is to also add a realistic change to the timber of the sound. Like playing a piano softly will have different overtones than if hit hard. 
Not all instruments take advantage of this and it depends on the extent of sampling. 
Bass VST's seem basic and I've never experienced any change in timber with the few I've used.  
I would try using the Velocity scaling set 100 and see if it works. Normally it always set all notes to 100 and they will all sound the same. If it doesn't then myself I'd blame the VST and complain to the company.  

I hjave PreBass open right now in a track.  The MIDI velocity does make a difference, but it does not seem to be linear.  The difference between 50 and 70 is more obvious than the difference between 90 and 110.

Thank you to everyone that responded. It was puzzling, but I figured it out (I think!) The volume issue arose when I was going from a "B" to an "A" note. So, apparently, Pre-Bass does a slide (legato?) between notes that are that close an interval. I need to completely release the "B" note before hitting the "A". It was a legato thing, which it seems loses the power of the "A" note that is slid (slided?) down to. 
 
Seems I have to exactly release the "B" before I hit the "A." 

Seems pretty realistic. If I pluck a note and slide to another, it would legato and the second note wouldn't have the attack from the first note. This is also quite normal on bass synths, where overlapping the note causes portemnto to be used and the portamento rate to set the speed of the change. This would bypass re-triggering the attack phase of ASDR.
 
You may be able to turn the off, or change the number of voices for triggering chords, or simulating the first note being on another string