There is no reason to use the 5-pin XLR unless you want the +and- 15V DC power for a preamp.
I recommend that you use the "power amp input" jacks to connect from your sound source to the FR-7710 amp. I think this schematic matches your FR-7710 amp: https://www.fenderrhodes.com/org/ch11/fig11-3.jpg. Zoom in to the upper left corner to see the input jack arrangements.
You will probably not be super happy with the Rhodes Stage Piano plugged in to these inputs, but if you put any kind of clean boost between the Rhodes Stage and the FR-7710 you can achieve happiness. You could buy a Rhodes preamp, or you would be perfectly fine with a little mackie mixer (or even a cheaper alternative). I like the mixer option, because it allows you to get EQ and as much gain as you will ever need, and it can provide left and right outputs from your mono source.
Let's hope you can get that amp running!
Sean