Do I get you right, Aria Player does turn up in Dorico and sound does come out, but MIDI channels are limited?
The “_x64” is certainly needed, because Dorico and the audio engine are only 64Bit. Could you please zip up the VSTAudioEngine folder. From the log files I can see what plug-ins get picked up on your system.
Here is the folder:
VSTAudioEngine.zip (241 KB)
It’s really weird now. Suddenly I can’t get the Aria-player to show up at all anymore. I’ve tried all possible combinations without any success. 
At the moment the Wihtielist looks like this:
Kontakt 5 16out
5653544E694F376B6F6E74616B742035
ARIA Player Multi VST_x64
565354474152616172696120706C6179
ARIA Player VST_x64
565354474152616172696120706C6179
And here is the defaults-file:
Defaults.xml.zip (2.65 KB)
This is how it looks in the Aria-Player now:
As you can see see all outputs but 1/2 are grayed out. Before I could change so that every instrument had its own output (eg. 1/2, 3/4, 5/6 etc).
Before I changed to a SSD and re-installed everything on my computer and upgraded to OS X 10.11 (El Capitan) it worked perfect.
Hmmm, I’m a bit lost right now…