I have one of those vintage Marantz that was my dad’s. I will have to look at the model number. Looks just like the one you have. They have a good sound/tone. Just keep it a use it unless you need more features or dislike the sound. Most of the improvement would come from the preamp section, not the amp IMO. Maybe demo some integrated amps.
the only amps I have tried with the Cornwall IV were McIntosh and the results were pretty average. Not sure why. I like McIntosh (I own some) and Klipsch ok (also own some of them too…).