Sorry, been offline for a while. Thanks for your suggestions!
Finally, it is working.
Attempt No.1 Removing all Line6 Software from the computer
All Software but Line6WinUSB Driver 1.0.0.0 unsinstalled cleanly. Ignored the later, re-installed Helix Driver 1.91
Set Helix in Windows as exclusive playback and recording device, disabling all others explicitly
--> still not working :(
Attempt No.2 Removing all Line6 Software from the computer and manually clean Windows Registry
Uninstalled Helix Driver from Attempt No.1. Deleted ALL Line6 entries from Windows Registry manually. Reboot and re-install 1.91. Reboot
Set Helix in Windows as exclusive playback and recording device, disabling all others explicitly
Start Reaper->Options->Preferences and Helix ASIO was shown as Default, with Ins and Outs 1...8 enabled automatically
--> everything working fine. :-)
I will gradually add other Line 6 Software now. Let's hope. Will re-post on this thread if new issues come up.
Cheers
Oli