Hi there murphy761,
thank you for your reply.
Indeed it made no sense to try and avoid using a router / switch
so I set it up using a TP-Link 8 port Gigabit desktop switch.
Connecting all macpros using their Ethernet port 2
I followed your tips starting with Auto at first, then moving on to Manually (it seems important to have a manually set-up IP for each machine so that master and slaves can connect and reconnect without being afraid of each other..)
I had until now never touched settings such as Advanced / Hardware / speed, duplex, link local only...
so I did and set them up as you mentioned.
I fumbled around for a few days like a madman, and have in the end, achieved great improvements:
No more pops and clicks. A buffer size of 192 in Cubase.
All my VEP Pro plug-ins are set to two buffers.
If an instrument is complaining, I set it to 4 buffers, and that usually does it.
It's a one master, two slaves situation, with Macpros.
So now I'm off to buy an iLok or two (sigh)