Hehe, I've actually done the opposite before. I can't play guitar to save my life, and my guitar's intonation is totally fed up. So I just play all the single note guitar parts in my home demos at half speed on my bass (thank god for Reaper's speed bar!) then playback at normal speed. I then either reamp it into my Music Man amp or just do Nigel or something.
That can be fun too! It'll let you fake some virtuouso shit.
My music is usually so slow already though that half time becomes very tough. In fact, there have been times when I was forced to record a scratch track at doubletime so I could have some
reference for the other tracks.
It's actually pretty amazing how much a varisped electric guitar really does sound like a real bass guitar. You lose the top octave - mostly just pick attack, string rattle, and "zing". It's very much like switching from a single coil to
a humbucker. Otherwise, though, it's pretty convincing as long as you play it like a bass. I can see where recording in double time might be a problem for the metal crowd!
If the OP ever comes back around - the MIDI coming out of
Melodyne says what note and (maybe? hopefully!) how hard. For that
reason, it works best to simulate instruments who's articulations are limited to those two parameters. That means keyboard instruments, hammered instruments (xylophone, marimba), and - to a somewhat arguable extent - drums. And synths, of course. To the extent that the sound you're trying to emulate depends on more parameters than this you will always be compromising. Well, I suppose it's a compromise begin, but if we're talking about minimizing the damage...
As long as you're happy with very simple, consistent, Bomp Bomp Bomp kinda stuff, you should be able to get pretty close with a decent VSTi bass guitar, as long as you treat that like the sound coming out of the bass itself. Compress and EQ the way you would with a real bass. Run it through an amp either in meatspace or ITB. And don't hope to sound like anything spectacular.
But we don't actually know what kinda music you're trying to make. You said you were trying to get the sound of a bass guitar. Maybe you can replace it with something else? The doors used an organ bass, which is exactly the kind of thing that MIDI does very well.
A Rhodes doesn't actually get down into
the rumble bass area, but it's lower register can sound pretty good for certain things. Or maybe what you really need is a 303?.
Heck, even Tuba and Baritone sounds can work as long as you play within their
natural range and don't get too fancy.
Anyway, there are lots of ways to skin a cat. I personally wouldn't use any of
the straight realtime pitch shifters that I own for any attempt at clean, natural bass. Some are better than others, but the good ones are very latent (only a problem if you want to hear it while recording) and they all impart nasty artifacts. One can embrace those artifacts as an effect, but it's definitely not appropriate for everything, and almost never good without some pretty severe treatment downstream.
Enough distortion will fix just about anything.
Edit - the title of this thread should probably have been "faking bass guitar". I think folks were excited to share tips and tricks. When I want to (and can) record a bass guitar, I plug it in and hit record.
Actually my real problem is playing all those other instruments properly. And as an afterthought, you never appreciate the bass until 