Correct, it goes between the two. You will want a pair of XLR cables to run from the ARC to the monitors.
The HS8 speakers sound a lot better with the ARC. At least for my room, it fills out the low end substantially, where it was more upper bass before, now it's a lot flatter and deeper. It also scoops out the low mids a bit which clears up the sound. I think for the most part that's due to room issues rather than speakers but either way it should help.
I've spent the last few days rearranging my room and trying different speaker positions. Funny enough every change I make seems to make it worse lol. But I'm getting my desk put back together now.