Yes, there are stereo RCA inputs on the back of the EO9K. There is also USB input, same place.
When you use optical > E17/E09K the optical plugs into the top of the (docked) E17.
Regarding the sound quality of E17 vs. E17>EO9K ..... (I began a reply to your PM but did not finish yet) I have had the E09K for about a week - so I don't have a lot to say, other than this: The short trial I did give it, plugged LCD2 into the front of the E09K<E17, the sound was good .. a lot of power (even on low gain setting), has a nice big smooth knob.
I heard nothing objectionable.* Which is to say, maybe, that it sounded like a higher powered E17. Remember it's been only a short session, but so far I'm really pleased with it. I find the E09K only enhances the E17 - I don't sense any added character to the sound.
*that's kinda my way of saying simply "it sounds great", i.e. 'transparent', 'accurate', 'I hear the Music!'.
My plan for the E09K is not so much to have another 1/4" jack (amp) to listen through, but more for the features it offers as an "inline" processor ... to EQ a recording, use the DAC for music on the computer, run the pre-out to powered monitors, run the other Out to another tube amp, etc., etc.
I may even get another one for the other room!
Another handy feature I'll mention, applies to both E17 and the E17/09K combo: I use them as a sort of "tethered remote" ... for instance, the "bedside amp" while close enough for the 'phones cable, is too far to reach without crawling out of bed.
With the Fiio combo on a nightstand (where it fits nicely), the line-outs running to the tube amp, I can adjust volume, tone, sleep timer, etc. right at the bedside. I did the same thing when I had the E17 alone.
Similarly, in the other listening room, my chair is not within arms-length of the component rack -- but I can place the Fiio combo near me, for easy adjustments.