Thank you all for your interest in my project!
I think I will use a new strategy.
Beginning from zero!
At the moment, my problem is that I don't really know how to set the buffer size so that all members of the band say "it's ok to play with this setting, I can't notice latency".
So, what setting should be good? 32, 64 or even 128 samples buffer size?
According to the blog of CrocoDuck, a roundtrip latency of about 5-7ms should be good when it comes to in-ear monitoring.
The Kemper and Line 6 Helix produce about 1ms latency.
The Behringer ADA8200 produces 1ms latency.
Configuring Reaper at 48kHz, 128 samples, 24bit and 2 period brings 2.6/2.6ms. It's means 5,2ms (in and out).
The roundtrip latency is also about 1+1+5,2 = 7,2ms.
So... The biggest buffer size allowed should be 128 samples.
You're advices are made of gold
I can remember that I've connected the RME RayDAT on a big 16x PCIe slot of the mainboard (normally intended for graphic cards, is it?).
The board connector of the RayDAT is really small and would also fit into a PCIe 1x slot.
I will try that PCIe 1x slot. Good idea!
The second ADA8200 (on ADAT ports 3 and 4) was not all the time synced. Sometimes the RME driver software in Win10 displayed a short "lock" status (shorter that 1 second) instead of the "sync" status. It was regularly switching from "sync" to "lock".
I will check that and maybe buy a new RayDAT. Or maybe was a TosLink cable not good?
The RayDAT was set as sync master.
Then, I will try to set up the software, probably from a vanilla Debian Stretch. I'm usually an Ubuntu user, so Debian is the distro I better know. I feel a bit lost on Fedora, Arch, Suse, etc.
Avoiding PDC is something I do whenever possible, since it can cause some glitches when muting/unmuting tracks or disabling/enabling plugins. When PDC is very high, it can make things a bit weird sometimes too.
I'm only for 2 days aware of the PDC shown in the Reaper's performance meter. I will try to avoid any PDC producing plugins.
To get state of the art Linux audio you're likely going to have to learn more Linux.
True, sadly... A lot of time before anything works...
So my tip the less plugins the better.
I will try with the same project and removing all plugins to see what happens.
Can you minimize the 50 plugins by (sub) groups?
Maybe.. But the Reaper project I set up is something realistic, tuned for our real needs, including the routing.
If you get a solution for your problem there please post it here and in the Reaper forum. Good luck!
Maybe in the coming weeks if the system (Win10 or Linux) doesn't become better.
I wonder if it isn't just normal. I mean 32 samples is pretty small ...
Some reported some success at these settings! And not only on linuxmusicians.com