A good ( I mean, a real good one) Phonostage simply delivers you more information from any connected cartridge. But, getting a good Phonostage (one, which does not sound like a dead stone) is much more difficult than getting a good sounding cartridge. You would be amazed how many carts sound really good, even a Denon 103R can give very satisfying results (overall probably better ones than Koetsus for example).
But, there is not only one way to Paradise. It is far easier to design a good MM-level Phono Stage than a stellar MC-level Phono Stage, implementing the extra 30+dB gain required at a high level of quality does not come easily nor cheaply.
Based on this many Listeners may get better sonic results with a high output fixed coil cartridge (MM or MI) into a low-gain Phono Stage than a low-output, low-impedance cartridge (MC) into many MC high-gain Phono Stages. The lower the output voltage from the cartridge, the more capable the Phono Stage needs to be ( Price & Brain from the Designer ). To Design a world class Phono Stage (I write that, because every owner of anything thinks, his unit is worldclass) is not a trivial accomplishment. And Designers do something because they want to sell...not to make a religion out of it. Marketing will do that job much better...
So, back to beginning, better Phono Stages are able to extract more of the performance potential of the cartridge. Any cartridge. When you have ,bad luck, you can also hear what carts are good or not. Not everyone is mad to get that kind of truth..