Well, this thread caught my attention while searching for this phenomenon on the net because I've experienced it myself, so it led to a full blown test with any variations possible on these systems:

OS 10.6.8 Logic 9.1.8 (32-Bit)
OS 10.6.8 Logic 9.1.8 (64-Bit)
OS 10.8.5 Logic 9.1.8 (32-Bit)
OS 10.8.5 Logic 9.1.8 (64-Bit)
OS 10.8.5 Logic 10.0.3 (64-Bit)

I was able to determine the cause of different sound as described below.

1. Imported a single audio file and bounced from the systems above to a complete new audio file. No plugins were active. Conclusion: Complete nulling with all versions: Each bounced file seems to be exactly the same.


2. Loaded a multitrack songfile (44 audio tracks) with no plugins active, completely blank. To my surprise, differences occured by null test. I couldn't believe it and re-bounced to be sure: Same results.

These bounces ofwere exactly the same and completely nulled within each Set. When trying to null Set A with Set B, nulling was not possible! What caught my attention is that on audio files that not null, always bass and subbass frequencies of the kick drum were prominent. So the difference seems to be in the bass region, especially kick drum thump. (Bass Guitar was completely nulled and not audible)

Set A
OS 10.6.8 Logic 9.1.8 (32-Bit)
OS 10.6.8 Logic 9.1.8 (64-Bit)
OS 10.8.5 Logic 9.1.8 (64-Bit)

Set B
OS 10.8.5 Logic 9.1.8 (32-Bit)
OS 10.8.5 Logic 10.0.3 (64-Bit)


3. Now loaded a mixed songfile with many plugins active which work on all systems (Used only 64-compatible versions to keep the 32-bridge inactive). To my surprise, the bounced files did not null and showed interesting results:
A. OS 10.6.8 Logic 9.1.8 (32-Bit) - Best and forward sounding, compact, vivid, in your face, more prominent in bass + mids.
B. OS 10.6.8 Logic 9.1.8 (64-Bit) - Bit softer with a tad less focused version of A.
C. OS 10.8.5 Logic 9.1.8 (32-Bit) - Bit more washed out than C. less prominent midrange.
D. OS 10.8.5 Logic 9.1.8 (64-Bit) - Like C and a tad more loss of the described effects
E. OS 10.8.5 Logic 10.0.3 (64-Bit) - The softest example of all. In comparison to A. it sounded like putting a very soft blur effect on the mix. Nice seperation, softer highs+mids, less bass, very hi-fi and not so in your face.
As described under 2. kick drum thumping is very prominent in the null test, as very slight stereo image bleed, i.e. with 10.8.5 Logic X and 10.6.8 Logic 9.1.8 32-bit.

Final conclusion: The more tracks and plugins, the more difference occurs. The OS and core audio, Logic's 32-64 bit versions and rendering engine seem to introduce different results.

Personally, I liked the OS 10.6.8 Logic 9.1.8 32-bit the best sounding one.

You're welcome to share your experience on this.