Hi
I do need help : I did a song based on a live piano + vocal , then I recorded a live midi bass on it and than a live midi drums.
As far as song tempo is changing a lot , i couldn't use quantize or clock to record bass and drums.
My question is : is there a way to link in a magnetic way the midi bass line and midi drums that I recorded live ? ( so that , e.g. ,  bass drum will be in sync with bass notes when they're quite close? )
I hope I explained it well
Thanks
Ufostraker

in short, without using quantize, no.
you would have to open them both up in piano roll and do a lot of hand work.
moving notes around to match eachother.

MIDI quantize works across tempo changes. Of course, you'll need to quantize all tracks so that they're all conforming to the same timeline. Where quantizing gets you into trouble is when you're  using too-fine a snap setting, which can cause the note to snap in the wrong direction.
 
Personally, I avoid quantizing because it's more trouble than it's worth. Just turn on grid snap to eighth-note resolution, display all MIDI tracks in the PRV and watch the notes go by as you play the project. If a timing error doesn't jump out at you while you're listening, it probably doesn't need fixing - even if it looks wrong on the screen. For the obvious timing flubs, manually drag the note into place.
 
If the drums were recorded live (as opposed to being sequenced), defer to them as your timing standard, not the timeline. Good drummers aren't quantized, and shouldn't be after the fact.

You could try this:
1) cut your long midi clips for drums and bass into smaller, bit-sized pieces where you know the tempo changes drastically
2) select the kick and snare midi notes in a drum clip and ctrl+c to copy
3) select the corresponding bass clip 
4) click on process/qroove quantize 
5) in the dialog box, just click ok
 
if the results aren't what you like adjust the resolution and window sensitivity.  
 

Thanks
I'll try
Ufostraker