I have a Combi and it is a simple full wave controller with no active feedback. For such a simple controller it works well, although for some mechanisms it may have a large "dead band". That is, you have to turn the knob up to 30 or 40 before anything moves.
I have recently built 12 DIY DC controllers of various designs and "tested" them to see how my N gauge motors respond. These are a 5pole (Graham Farish GP tank) , a Dapol 45XX and a Kato tram with coreless motor. The Dapol and particularly the coreless motor were unhappy with any PWM controller, although most of these had feedback. Slow speed, smooth, jerk free performance was most important for me, so the "Cooler Crawler" design was my best because it worked nicely with all my 3 motor types
Anyway, have a look at the 852B Varipulse. It is available built, kit or buy parts and make it all yourself (I did this on vero board).
http://www3.sympatico.ca/kstapleton3/851.HTM
I've think I've seen another recommendation for the 852B on this forum.
Cheers,
Mark