Re: M-audio firewire solo setup right ?
Your system specs are more than adequate. I'm not familliar with Guitar Rig, is that a VST instrument? If so, that could be your problem, running 6 VSTs in real time is going to eat memory, since I'd imagine guitar rig is a pretty heavy going program. Have you tried exporting the tracks to .wav with the VST, then put them back a new track so you can play them dry? That should tell you whether or not it's guitar rig slowing it down.
That was a pretty terrible explanation, but I hope you understand what I mean!
Do you have a sub with those speakers? If so, that should have a power amp in it, try connecting to that.