The Raspberry Pi came back to me…
The RPI3 is running a Ubuntu Mate on top of a 4.4.7 linux kernel with full preemption. First I tried to get the zynaddsubfx running with a graphical system (desktop) but it works much better with a headless system and all unneccesary daemons running in the background stopped. We needed also to do some tweaks to optimize the system.

To get access to the soundmachine we connected via ssh and started qjackctl and zynaddsubfx from the commandline, enabling access to the synth via osc.
Back on a local linux machine we start the zynaddsubfx external gui which connects over the network to the Raspberry Pi 3. It allows us to controll everything from our local machine without the need of running a resource hungry X system on the Raspberry Pi.

Here is a little video about the result…