There is no rule and no single answer to your question. It is all system dependent. I run one system with 4 mono amps, biamping each speaker, and I run another with a stereo tube amp on it's own.
I'd rather have a good sounding stereo amp than a pair of not as good sounding mono amps.
I'd rather have a good sounding stereo amp than a pair of not as good sounding mono amps.