There's not an answer to your need. The best scrubber is probably the Cimex (from what I read about it), but as you say, it's too heavy. The next best, imo, is the 175. I own a 175, an OP and a Whittaker GLS. The 175 is the best of the three because it's weight makes it scrub better than a CBM like the GLS (if you run a brush on it), it's speed makes it better than an OP (which is slow as hell), and it's lower cost makes it more affordable.
However, it's weight is a pita for old guys. Of course, if you put larger rubber wheels on it, it can go up stairs fairly easily, if you slow down enough to actually step it up stairs, instead of carrying it.
The CBM machines give you the illusion of better scubbing because of the fiber balls it will dig up, but it's light weight makes it skim across the top of the pile instead of digging deep. A side by side test will reveal a big difference. The soil will almost completely disappear when scrubbed with a rotary, while the GLS will leave spots still visible.
Bear in mind ... the GLS (and all it's imitators) was designed for low pile commercial carpets.
You'll hear CBM machine owners argue that their machines are better scrubbers, but you won't find one who owns a 175 WITH A BRUSH ON IT, and the balls to actually use it.
