The most effective scrubbers for carpet are the counter rotating brush machines. There are several types. Host was the first to come out with one. Used mainly for their sawdust cleaning system they work great for aggitating prespray. I started off using one and it did get me hooked on scrubbing carpets before they are extracted. The problem with them, very slow. The motor on them is not very powerfull and the cleaning path is only 12".
There is another type that powerflite sells and others, cobb Is one. It is a powerfull
crb scrubber and I have never used it. The problem is weight. The thing weighs 80 pounds or so.
Finally I come to my favorite. The whittaker
crb scrubber. Original one was called the gls scrubber which is an all powder coated white collored machine. The newer ones are called lomac and they are a powder coated and stainless version. They work amazing well, are only about 48 pounds and have a 15" or 20" cleaning path. We use them every day. I havn't even had one break on me yet. Customers are very impressed the way it scrubs and lifts the fibers and it makes extraction much easier. There is one on ebay right now btw. Search whittaker Lomac it is a 20" version and weighs 5 pounds more or so.
For my company it really is the perfect fit for performance and usability. The weight thing is a very big deal when you consider going up stairs and lifting in and out of the van 4 times a day. IF a tool is too much effort to use it won't get used. Then it has no value.
As a side note many other companies actually brand the same machine which is made in austria. A company called brush and clean sells one, hydroforce sells one (available through interlink) and I think a company called mist and brush sells one. I have never purchased a new one, I have always bought off ebay or craigslist.
Another way we use ours is for encap cleaning the edges. we use a cimex for the paths and then bought an edge brush that fits the lomac that will dig out the lint and clean the edges quickly.
Pm me if you have any other questions about them. I really am a big fan as is my customers