It seems the Akai S950 will only work with a serial MIDI interface in the context of a successful MIDI sample dump (not note on/off) The S950 uses its own sample dump protocol, different to standard MIDI sample dump.
The following is info as I have it from various sources....
Alchemy is the best bet for sample dump on a Mac with the S950. Two way sample dump send and receive works fine if using the Apple MIDI manager with Alchemy, but this can only be with a Mac system 6 or 7. The reason for this is the S950 requires the MIDI sample dump sent to it at a certain speed which the Apple MIDI manger allows.
But if using OMS with Alchemy (Mac system 8 + 9) then yes the S950 will work, BUT only for one way transfer from the S950 to the Mac. As noted the FB post said this was only possible with a serial interface. I can partly confirm this as using my USB Edirol UM880, Alchemy lists the samples in the S950, but when I try to receive, the "blocks remaining to be transferred" dialog will appear, then show a block size number and then it takes around 10 mins for that number to decrease to the next block size number. So a sample of 73 samples would take many hours. I could not transfer a sample from Mac to the S950. Could only partially get it to work with USB MIDI, but the transfer would take hours. For some reason the S950 is not playing well with USB MIDI.
The guy on FB is using Alchemy, OMS, Mac 9.2, with an Altech MIDI interface, and shows that this works for S950 to Mac. So confirms serial will work, but as I found USB MIDI does not work.
The attached screen shots are showing Wallstreet, OS 9.2, Alchemy, Altech serial interface receiving a sample from the S950. (note the Altech interface is a non powered one) He tried the same with a serial AMT8 and Unitor and transfer did not work. Same guy confirmed that two way transfer works ok but only with Apple MIDI manger on OS 6 - 7.
I'm only needing one way send from the S950 to the Mac, as I'm finding a workaround for the lack of waveform display on the S950. I like nice crisp cuts on breaks, which I can do with Recycle and Akai S3000. So if I can dump the sample from the S950 to Alchemy, then open that sample in Recycle, I'd be able to see the exact sample number on the Recycle split point to type into the S950 for a nice clean cut. As I could not get the S950 to play with the USB Edirol, and it seems that a serial interface will work, my bet is a Keyspan serial and a Midi Macman powered version should work with my G4 PowerBook Ti... fingers crossed.