Thanks for your response.
Ok. So the initial plan will not work. Can anybody then recommend an external dac (not firewire) with 6 analogue outs and decent output stage at a reasonable price, that can be used for such a setup ?
I use the Steinberg UR824. It's a USB device, has 8 balanced outputs, and an excellent output stage (116dB SNR). It can be found for $700 if you hunt around for a sale (usually it's around $800). Focusrite makes an 8 output device (called the 18i20) that's going for around $400 on Amazon. I can't vouch for the Focusrite personally, but Matt (JRiver's CTO) has one and had some positive things to say about it. The Focusrite is the cheapest 6+ channel external interface with balanced outs that I'm personally aware of, but I'm sure there are some out there that are less expensive.
If you're willing to go internal (and unbalanced), there are 8-channel internal soundcards available much cheaper from Asus or Creative (an Asus DX has 8 unbalanced channels out, low rated distortion, and is around $80, for example). There are ups and downs to internal cards (driver issues, EMI, etc.), but they're by far the cheapest solution. I have two Asus cards (the DX and the ST/H-6 combo), and I still use my Asus ST/H-6 combo on my second PC.