Cactus Music
I see they have demos so I'm downloading right now. 2.3 GB wow. Glad I'm using someone elses bandwidth :) 
 
It is a mystery why Some VST's do not use GM standard pitch. 
And it seems to be a Bass thing as well. I use a lot of Dim Pro Acoustic Bass and it also needs the +12. 
All my old midi files I made myself have to be transposed. 
 
Anyhow I will test this myself later and post. Showing 1 HR to download. 

Thanks much, John...      one other thing I've noticed is that the timing of the original bass line and the midi version are not the same for some reason, and it is audible when you mix the two tracks. I've gone as far as quantizing the Melodyne track to 1/8 notes, just to be sure notes are on the mark, then pulled the clip into SI - Bass for the midi version, and they are slightly out of time, so this is lots of trouble that I guess is beyond me at this point. Dunno. 

For the ample basses and guitars the MIDI notes have to be shifted 50ms ahead to have correct timing. That's what they say in their manual:
 
After  pick  strokes  string,  it  takes  around  50ms  to  get  the  string  really  vibrating. Ample  Guitar
preserves the moment of the stroke, otherwise it will sound like piano.
 
Additionally you have to put the START time to 0 ms for play back (default is 35 ms). If you do both things, then the timing should be absolutly correct.
 

Chevy
But why doesn't Ample Lite work the same way that SI - Bass does?   I do the exact +12 transposition, and it doesn't work. (and why do you have to transpose it anyways?)
 

The reason for that is very old. Guitar and bass notes are usually written one octave to high. They wanted to use the standard violin clef for guitar scores to avoid the necessity to have always 2 clefs (guitar notes would be in the bass and the violin clef). And so some guitar and bass VST provider use the writing pitch (one octave to high) and others use the real sound pitch, i.e. the chaos is perfect. (By the way it is the same thing with male vocals!)

Well I just tried it and it plays in the same range as SI Bass for me,, No problem..Are you sure your selecting all the notes before you transposed?  It's the only reason I could see your issue happening. Use CTRL A to select all. 
And you can always drag the notes up to where you want them. 
 
One other thing is some VST's don't like 88.2 Sample rate. I'm using 44.1. 
 
Anyhow thanks for pointing this out. I've downloaded the full version demo, the Lite version and the Upright Bass. 
 
Can't really hear any difference between the full and the freebie. I guess the freebie is missing a bunch of features but so what, sounds good.  I really will use this one. A little twangier than SI.  The Upright bass doesn't sound as good as the Dim Pro patch I use. Good stuff though. Might keep an eye out for a sale. I just love freebies that sound good :) 
 
I'm in Electric piano heaven with my free copy of Addictive Keys and the Mark one freebie from Focusrite. 
 
 
 
 

Cactus Music
Well I just tried it and it plays in the same range as SI Bass for me,, No problem..Are you sure your selecting all the notes before you transposed?  It's the only reason I could see your issue happening. Use CTRL A to select all. 
 
 One other thing is some VST's don't like 88.2 Sample rate. I'm using 44.1. 
 

OK, tried it (on Ample Bass ) again, and it "does" work... sorta.  The lowest note Eb3 that I need for my stuff doesn't work....  if you look at the Ample keyboard, that Eb key isn't mapped for notes, it's mapped for doing nothing, it seems.  So E is as low as it will go.  If you're in Eb, you're out of luck it seems. If I could access keyboard mapping I guess I could fix that; or in the edit screen, I could perhaps retune the guitar down to 415 pitch from 440? Dunno. 
 
Too bad, this thing sounds good to my ears!
 
 

marled
For the ample basses and guitars the MIDI notes have to be shifted 50ms ahead to have correct timing. That's what they say in their manual:
 After  pick  strokes  string,  it  takes  around  50ms  to  get  the  string  really  vibrating. Ample  Guitar
preserves the moment of the stroke, otherwise it will sound like piano.
 
Additionally you have to put the START time to 0 ms for play back (default is 35 ms). If you do both things, then the timing should be absolutly correct.
 

OK, this helped...  thanks.   I still can't use it due to the muted lowest Eb problem, but came close. 

Here's a freebie I use a lot for scratch bass trax... sounds great!
 
http://www.yohng.com/software/bass.html
 

SI-Bass uses sfz files to map notes to samples (like all the sfz engine synths). While there is no way to alter the programs, it is possible to alter the sfz files loaded by the programs.
 
The default location for the sfz files used by SI-Bass is "C:\Program Files\Cakewalk\Studio Instruments\SI-Bass Guitar\Multisamples" or "C:\Program Files (x86)\Cakewalk\Studio Instruments\SI-Bass Guitar\Multisamples"


Here is an image of the top of "picked bass.sfz." The lokey key value is the one that needs to be altered.

Sfz files are text files and may be edited in notepad or any text editor.
 
The trick is figuring out which sfz to alter. While the program files are binaries, when opened in notepad program files will display the sfz file name (just don't save the file or make a copy of the programs just in case). IIRC, the SI instruments only work with the file name and ignore the path using the default path above.
 
The programs are located in "C:\ProgramData\Cakewalk\Studio Instruments\SI-Bass Guitar\Programs"
 
good luck

There are only 10 SI-Bass sfz files. Rather than mess with reading the prog files. edit all 10 of the sfz files and change the lokey to zero for the lowest pitch sample in each file. This will extend programs all they way to the lowest note possible. Don't expect the synth to sound good that low but it will solve the low end range problem and should be acceptable for the range mentioned in this thread. Make copies of the original files, just in case.

scook
There are only 10 SI-Bass sfz files. Rather than mess with reading the prog files. edit all 10 of the sfz files and change the lokey to zero for the lowest pitch sample in each file. This will extend programs all they way to the lowest note possible. Don't expect the synth to sound good that low but it will solve the low end range problem and should be acceptable for the range mentioned in this thread. Make copies of the original files, just in case.

Thanks, Scook...    I wan't totally clear I guess on my recent posts....   i can get SI-Bass to work down to Eb, that is fine.  The problem was trying to get the nice-sounding Ample Bass Lite to do the same.