I run my mixing desk into my hi-fi amp without any such issues, but I would say you should use an aux input on the amp, and not the phono input.
If your keyboard mixer has the higher 'pro' +4dBu output, there is a chance that it might overwhelm the theatre amp input if running the mixer into the red (like DJ's love doing). You can get in line attenuators, but as I said, I've not had issues myself.

Providing the all the equipment's power is sourced from the same point, you shouldn't have any ground loop issues (so no hum).
Even if despite your best efforts the dreaded hum does appear, a ground loop isolator will resolve them.
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