It is not at all difficult to design an amplifier with better than 50dB separation, even using valves, and any more than this is of no audible benefit.
Why monoblocks?
Why run monoblocks? Most amps can handle most speakers, especially those that are 88DB and above in sensitivity. I see pictures of all these speakers being driven in monblock configuration. If the stereo amp can do the job, why spend money on two amps? I mean, what are the benefits?