8-bit? That's your problem right there. Curious to know why you were bouncing at 8-bit.
As far as autotune goes, if you were running some kind of demo version then prolly your demo time is up, and now it's time to purchase the full-fledged plug. Other than that, you would know best if you deleted files that had anything to do with autotune.
Standards for music files:
AIFF or WAV are the standards. They're not even "professional" standards so much as plain "standard" file types. Sound differences? None. No difference between them. Why you'd want to bounce as one over the other has to do with various technical requirements that I won't get into right now. But save to say that if you want your bounces to sound [i]exactly[/i] like what you're hearing out of Logic, bounce to AIFF or WAV.
Now, on to bit depth...
Again, to bounce out exactly what you're hearing in Logic, set the bit depth to 24-bits. Now, if you're going to burn a CD, things change. CD's can only record 16-bit audio files, in which case you'll have to bounce to AIFF or WAV at 16-bits and enable dithering (POWr1 is a good choice). If you skip that step then you won't be able to burn those files to CD.
Now, on to MP3's...
MP3's are a standard file format for playback on iPods and stuff. They will only sound as good as the original Logic project if you bounce them at a high bit rate, like 256K or higher. Otherwise, they will inherently sound different (if not worse) than their 24-bit (or 16-bit dithered) AIFF or WAV brethren. And that's because MP3's are made by processing the audio through an algorithm which attempts to remove "unnecessary" parts of the overall sound. So what's necessary to you and me might not be "necessary" when the MP3 is getting made (especially at low bit rates), ergo such low bit-rate MP3's will not sound like your original production.
As far as other file formats are concerned, I wouldn't bother with them. The only ones which, IMO, you need to worry about are AIFF, WAV, or MP3.