I recently purchased a new 2013 iMac 27" with the 3.5 GHz I7, 3TB Fusion Drive, GForce GTX 780M Graphics and put in 32 GB ram from OWC. I also picked up a Focusrite Scarlett 6i6 USB audio interface. I did a clean install of OSX Mavericks on the new iMac. I installed Logic X, Reason 7, Ableton live 9 light and Finale Cut Pro X.
There are issues with the Scarlett 6i6 and Mavericks, Focusrite has some temporary work around for this while they figure it out. While researching the issues with the Scarlett I ran across several mentions that the Focusrite Saffire Pro 24 was working well with Mavericks.
To cut to the chase, I ordered a Saffire Pro 24 DSP last week and on Monday I ran across this discussion group, was not a happy camper. The new Saffire Pro arrived yesterday and I took it home and hooked it up installed the firmware update and Saffire Mix Control. I turned it on opened iTunes and started Dire Straits and I had audio! However it only lasted about 10 seconds. If you've read this whole discussion this is not a surprise.
Testing part 2: I have a early 2011 MacBook Pro at work and I brought the Saffire Pro with me to work today. I am running Mavericks on my MacBook and the same audio and video programs as well. I hooked up the Saffire Pro to my MacBook Pro, the same way as I did the new iMac, Firewire to thunderbolt adapter and took another run at Dire Straits and again I had audio, and after 10 Seconds I still had audio, after 45 min. I still had audio. I switched to Ableton Live and went through the instrument tabs they all played fine. Opened Logic and no problem.
The internal audio on the new iMac works fine so what ever the issue is there I believe is fixable. I don't think Mavericks is the problem with the Saffire as mentioned above, sounds like maybe Fusion drive is the issue. I'm going to hook up an external bootable hard drive and see if that takes care of the iMac as a temporary fix.
The fact that the internal audio works with the Fusion drive indicates that it's doable. I hope Apple and Focusrite can get it worked out soon......