You should be able to run monitors off the monitor output, this will be either the main mix or pfl (pre-fader listen) per channel. Plugging phones in should cut the monitor feed to the speakers.
At least that's how my Soundcraft FX8 works and I'm guessing the M8 is similar. You can download pdf of all the diagrams and manual from Soundcraft which is what I did before deciding to buy the FX8, they show exactly what goes where.
My FX8 has a couple of simple stereo inputs - just a level knob, and I use one to listen to the main stereo mix from the soundcard. You should have the choice of channel direct outs, inserts or aux sends to use as record sends to the soundcard inputs. You cannot use the main (analog or s/pdif) as record sends as they only carry the main stereo mix, not selected channels. This is why I ruled out the M8 for my home studio, it's intended for live use with the option to record the mix off the s/pdif. The FX8 (or SX12) options have extra stereo "sub" bus that work well as record sends to a 2in card, as well as directs from the mic pres (pre or post eq) for cards with more channels.
Maybe someone is using an 'M' in a soundcard studio, but I couldn't see it working well.
If it's s/pdif output is tempting you, I can find no specs that reveal it's quality, and it could be lower than a reasonable soundcard like the Audiophile2496.
