I think ProAc monitors are a great choice for small rooms. And you can't really go wrong with any of the speakers you listed. If it was my money, I'd get the 8 Sigs over the Ref 8s, with the 1SC and 1S coming in 3rd and 4th. The above poster is right, the 1S series gives you more bass, but you lose some of the magic of the Sigs IMO. I'd say the 1SC has an extremely loyal niche following, but you'd have to listen to them to establish if you fit in that group and it's worth the extra money.
Both Ref speakers have a large soundstage, especially for monitors. And have a more natural sound than the 1S's., with the 8 Sigs having greater depth. I really think the phase plug gives you a more open sound as well. They also are a little more dynamic. Again, they both are amazing speakers for the price. If money is no object, I'd definitely get the Sigs. If you wanna save a few hundred, get the Ref 8s and don't think twice. You really can't go wrong either way.
Both Ref speakers have a large soundstage, especially for monitors. And have a more natural sound than the 1S's., with the 8 Sigs having greater depth. I really think the phase plug gives you a more open sound as well. They also are a little more dynamic. Again, they both are amazing speakers for the price. If money is no object, I'd definitely get the Sigs. If you wanna save a few hundred, get the Ref 8s and don't think twice. You really can't go wrong either way.