In the mid 1990's Audio Research was trying to go more mainstream. To that end they designed their gear for a lower price point. The big black boxes. I remember that the United Audio by me was carrying their equipment for a while.
I have heard the Ref 300s but it was a number of years ago. I remember thinking they were very good but I don't remember any specifics. It was at the dealer and not in my system. They were way out of my reach at that point.
More power does tend to sound better. I bi amped a pair of VT100 mk3s for a while and it was an definite improvement over just a single VT100. But I am not sure how much of a difference there would be between the Ref 300 and 600. It would also depend on how efficient your speakers are. The 210s are much newer and even with more power I am not sure the Ref 600 would be better
If it were me I would choose the Ref 210s. They truly are great sounding amps. Mated with a Ref 5 they would just sound incredible.
Nolitan, The VS115 is a very nice amp but not as good as the Ref 110. Then again the Ref 110 is not quite twice the price of the VS115. I have always liked the idea of having my amp and preamp from the same manufacture and from the same time period. They were designed by the manufacturer to work well together. From a manufacturer such as Audio Research, it can give your system a wonderful synergy.