"modding" an onyx card really is the best way forward I believe.
There are 3rd parties out there that are building OEM cards for 1394, but it looks like these cards are more expensive than the onyx card itself.
But... and this is the bit I don't like....
The converters in the card (i suspect they are int he card rather than the mixer) are pretty shoddy. From the Mackie website:
> 98 dB, 20 Hz to 20 kHz @ 96 kHz sample rate
> 100 dB, 20 Hz to 20 kHz @ 44.1 kHz sample rate
That sounds like low performance Codec's or sub $1 multichannel DVD player DAC's. Which is fine, as long as that's what you want.
It would have been better if they could have integrated higher performance ADC's or DAC's into the product (e.g. TI, AKM, Cirrus or Wolfson).