When you find a stable OS with your hardware, I'd suggest not messing with a good thing and go with what works. That's what I do. I have a Mackie system running on 10.4.7 on my partitioned hard drive. On the other half of the drive I run 10.5. No problems.
That's also a solution that costs 0 dollars to fix.
If you visit the Mackie site they outline what hardware works with what version of the OS. They suggest a roll-back to 10.4.7, actually. That may not make you happy as you may want to have the newer OS, but if it works, it works.
When folks go outside the bounds of the recommended configurations most likely they'll have problems. Now, if you WANT to basically beta test gear and futz with settings endlessly then that's your prerogative, but I must admit I'm amazed that folks constantly upgrade their OS software when it causes nothing but operational headaches.
By your own admission the gear worked fine with a previous set-up. What's your motive to essentially break your working set-up?
Is having 10.5.6 essential to your creative workflow?
Good Luck.