As you realize, the MA2275 incorporates a phono stage. I looked at its specifications at the McIntosh site, though, and it looks like it is suitable for moving magnet or moving iron or "high output" moving coil phono cartridges, but not for "low output" moving coil cartridges. I'm basing that on its signal-to-noise specifications for the phono input, which suggest that hiss level would be excessive with a low output moving coil.
If you choose a low output moving coil cartridge for your turntable upgrade, and that is a very common choice with high-end turntables, you would have to use an external phono stage that is compatible in terms of gain and signal-to-noise performance (i.e., one that is intended for low output moving coil cartridges). You would route its output into an aux input or other line level input of the MA2275.
I'm not particularly familiar with either the KD990 or the Scout, but I know that a lot of people here use the Scout, and it may very well represent a significant upgrade. Hopefully others will comment on that.
It is always highly important, as you may realize, to be sure that a phono cartridge is a good match for the tonearm of the turntable it will be used on, and if you decide to upgrade you'll probably want to start a separate thread on cartridge selection. The key criterion is that cartridge "compliance" has to be a suitable match for tonearm "effective mass."