This question (multi-channel audio from a PC) crops up reasonably frequently.
Frankly, I would ignore SPDIF/optical completely; it's good for 2-channel only. I've concluded that there are two routes you can take the audio out of a PC.
1. HDMI.
2. Multi-channel analogue.
It's unclear to me whether you want to mix in the Phantoms with your Meridian speakers. If you do, ignore the following; you're into a custom solution.
To solve this for your 4x D600 and M60C, I would choose one of the following:
For option 1. (arguably the better sound quality option even though HDMI is involved), I'd use an HD621 and 568.2MM combo.
For option 2, something like a G6x which has a multi-channel analogue input.
I'm only stating Meridian-only solutions because you are talking of using DSP loudspeakers and also have an analogue centre in the mix.
Neither is a "cheap" solution, but both are under £1000.