The 990 is my first pre/pro. The unbalenced inputs look like regular RCA connections. I was thinking of using regular Monster THX RCA cables. Is this sufficient? I don't know if there is a difference between RCA and XLR cables. Any advise would be greatly appreciated.

If it were me, I'd go back to Outlaw for their own interconnects. As good -- maybe even better -- than Monster at a significantly lower cost.

RCA cables carry an unbalanced signal. By "XLR cables", I presume you mean cables with XLR connectors on the ends. They are designed to carry a balanced signal. You'd need amps with balanced inputs to use the balanced outputs on the 990 (or make an adapter cable, but it would be easier to just use the unbalanced outputs in that case). Google is your friend if you don't understand the difference.

Monster's cables are generally overpriced, in my opinion. Outlaw's cables are very nice (I use them for many of my analog innterconnects), as are Blue Jeans Cables. Also, for the difference between RCA and XLR, this page might help.