A good way to make midi parts sound real , play the part on a good midi or ? instrument.
Example play drum parts on a digital kit instead of drawing notes or poking at a keyboard.
My first backing tracks I programmed my Roland 505 and then recorded the midi output to my Atari.
I used those patterns as the basis of all my tracks. Talk about a machine drum part for you, 100%.. At first I didn't care and I don't think the punters noticed either. But is soon drove me bonkers.
I soon bought a midi keyboard, A Poly 800, but it only output Velocity 64.
Later I bought a Roland PC 100 and now I had velocity, so drums are starting to sound better.
Now I have a Yamaha DX 450 kit and things took another jump in realism.
Next would be a better digital kit which you can pretty much achieve close to real.
Even if you suck as a drummer ( me) you can just go for it and edit the results.
now the bass.
I soon learned how to play bass parts on the Poly 800. I'm a bass player with a million hours stage time, My keyboard bass parts were far short of what I could actually play on a real bass. I slowly improved my chops but there that mechanical feel, all perfectly placed, quantizied and velocity 64. No soul.
I bought a Roland GR50 guitar synth hoping to play bass to midi. It didn't really work as back then storage was an issue and the GR50 cranked out huge amounts of Controller data filling a floppy disk with only 3 songs! And you can't really play bass on guitar.
Along comes Sonar and now I can record real Bass and just in the last year I discovered how to drag and drop that audio bass track to a midi track and we now have what you are wanting. A pretty authentic midi bass part.
So there's one tip, grab yourself a $ 100 bass, It barley needs to work, Record the audio, drag to midi, edit and assign to SI Bass. You'll have a pretty convincing bass sound.