NECROPOST!
Wow, this is one of the very early SFD-2s. The SFD-II Mk.III I'm listening to right now was an original Mk.I SFD-2 as well. My case is rather different - rather than just the top coming off it's a top and side type arrangement. Mine says 1994 on the analog board near the output jacks.
Apparently the analog board changed a bit over the Mk.I run. If you bought a brand new Mk.II, you have the latest analog board. Some Mk.Is have the same but only the later ones. I'm not sure that the Mk.II upgrade changed much other than the digital filter swapout to PMD100...
The Mk.III was available as an upgrade only as far as I am aware. The Mk.III added SF's funky I^2-Se input (output mod available for SFT-1 and standard on Transport 3, and Assemblage D2D-1 upsampler). Mk.III uses CS8414 receiver for all but I^2-Se, PMD200, PCM1704s (eight of 'em!) and OPA627s for I/V...
The Mk.III is fucking great... and insane with the D2D-2 via I^2-Se... (changing my redbook life, i'm in heaven! you know that face Fremer's pulling in the gizmodo article listening to Avalon? that's me right now but with Stax on, and for only 5 cents on his $350k dollar and instead of a big black vynil disc it's the fabulous redbook layer of the SACD version of this album) my only complaint is the bass (both quantity and quality) falls short compred to my Pass D1 (24/96 upgraded unit with CS8414/DF1704/8xPCM1704). Chris Johnson tells me the SE+ upgrade should more than address that and I'm going to have Bursons put in place of the OPA627s as well... just waiting for funds to materialize...
As for digital cables, I have a SFT-1 (sans I^2-Se output...) with ST-optical output and have used it extensively with my Pass D1, and a bit with the SFD-2 Mk.III (rocking a Micromega Drive 3 mostly these days). It's definitely got its own sound, different than coax or xlr... I've only got one ST cable, a grey plain looking AT&T jobbie. Aural Symphonics would be happy to sell you a $1k "Optimism V2"... anyway, I find ST and XLR to be opposite sounds with coax a happy medium. XLR = smooth, laid back, good in the extremes, kind of fat and slow PRaT wise. ST = awesome midrange, wicked PRaT, weak in the extremes. coax = great balance of the two. This has held true with all the cables and transports I've used...