Unless I'm misunderstanding you, it sounds like your highlights are always blown out -- both in color and b&w -- unless you underexpose as you've described. If that is correct, it sounds more like digital sensor limitations rather than the processing software.
I almost always underexpose with digital sensors. 'Treat them like slide film in the old days where we exposed for the highlights as opposed to negative film where we exposed for the shadows and developed for the highlights. But I always attributed it to the limited dynamic range of digital sensors, not the processing software.
If, on the other hand, they are just blowing out during the conversion to mono, I'd love to see some examples if you have any you can share, of the correctly exposed color and blown out mono.