Regarding the accompaniment instrumentation on the 850 vs. 500. There is a big difference, try it out for yourself. I believe the issue is not the XG voices, but the soundboard in each unit. To my ear, the pianos, strings, brass instrumentation sounds so much more authentic coming from the 500.
tried
On placement, there are a lot of possible solutions. I first tried leaving the 500 on the right then adding the 850 bracket (with minor modifications) to left side of the keyboard underside. Ultimately, I found this to be too crowded and not very ascetically pleasing. Instead, I built a little black cabinet out of off-the-shelf materials from Home Depot, put both head-units on different shelves, added a Sonos Player (1. so I don't have to string Ethernet cabling, 2. to be able to play all my .wav DK songs through the Sonos controller/playlists/shuffle/etc.)
The box is mostly out of site, but easy to reach, and I use the iPad/Pod controller or Sonos for most of my DK music selections.
Another suggestion: I plug a high quality surge protector into a remote outlet and plug all of the DK/Sonos/monitor speakers into the surge protector not only to protect from spikes, but this allows me to remotely turn off the "mains power supply" to the DK, which I understand helps reset the master unit. This also comes in handy during wind/lightning storms to make sure no electricity can reach any part of the piano (or when I'm away). I always make sure I turn on each component in this order (and off in reverse). 1. DK 500 head unit (wait for boot), 2. DKC 850. 3. monitor speakers.
To get extra powerful playback of the accompaniment instrumentation, I run a mini-jack from the back of the headphone unit (not all DK's have this), to my stereo amplifier which allows me to change the volume levels of both the amplified piano and the accompaniment instrumentation independently. This is not only great for the piano's XG voices, but the accompaniment that comes from Smart Pianosoft and Pianosoft +CD audio albums.
Happy listening.