This answer from the Mackie d8b forum:

"if you're feeding the FX with digital audio, you'll still be passing it the embedded word clock in the audio signal, and if there is a parameter to set digital in as the word clock source for that device, you should do so. I feed a Lex PCM91 and TC M-one and Finalizer96 this way in my studio.
You'd get more benefit from your external clock device if you returned the FX digitally as well. I do this with a PDI card in the ALT slot.

....... the PDI card is the only way to do what you want to do.
You'll need a DIGITAL d25 break out cable, which will terminate with XLRs(four stereo in, four stereo out).
You can make an XLR to RCA adaptor with a few inches of analog cable to connect this to your SPDIF devices, and set these outputs to consumer in the d8b Setup window.

I see over 110 reads but O responses. would guess the interest is there so I'll further offer this. You can in fact run all BNC out of the Big Ben (in my studio) into the d8b, HDR2496, and DBX356, the eclipse has Optical (1 on BB), the TCFinalizer has AES, the Lexicon has Coaxial (1 on BB) and I understand there is a "double-dipping" method used on the Big Ben as it has 2 pair of AES/EBU which could allow, say a global FX unit like the TC or an Alesis Masterlink audio to be run digitally IN and OUT using the Big Ben and back into the d8b. This setup would only send clock to the fx units and the audio would then be run analog. John Reynolds definitely feels it is optimum to run all fx audio digitally but this requires proper cabling and port access per studio set up. The BB offers a total of 6 BNC outs and (only) the BNC OUTs show termination status on the front panel.

I'll also add this from Mike Rivers regarding termination:

"When stringing devices together on a word clock chain, what you want to do is have the 75 ohm termination on the LAST (and only the last) device on the chain. The HDR word clock input gives you a choice of terminated or unterminated but the d8b's Apogee word clock I/O card is permanently terminated (though I don't recall that the documentation tells you that anywhere).
So if you connect a word clock output to the d8b alone, then it's properly terminated. If you connect a word clock output to the HDR alone, you should push the Termination switch in (terminated). If you wanted to drive them both from the same word clock output, you'd put a T on the HDR word clock input, leave the termination switch on the HDR out (not terminated) and connect the "output" side of the T to the d8b, where the termination is already there (no external termiator needed). If you were to put the T on the d8b and connect the word clock source to the d8b and then on to the HDR, you'd be doing it wrong. If you terminated the HDR (being the last device on the string), you'd double the load on the clock source, and if you switched the HDR's termination off, you'd have the termination in the middle of the string.
If you have a device like the Big Ben, with multiple word clock outputs, you should connect one output to each device. On the HDR, switch on the termination. On the d8b, it's already terminated. If you were to connect an output to anything else (a digital processor, for instance) you'd need to check its manual to see if it was internally terminated (if it's not obvious because it has a switch like the HDR). Most devices are internally terminated because they assume that you won't be chaining the word clock to other devices."