Hi Leiven
Yes I have the same problem with one preamp socket (B5) where increasing the gain past about 19db causes a big jump. I contacted local distributor about iit a long time ago and they said to return for repair as it is a hardware fault. I have not done that as yet – I just work around it as you have by either assigning another socket or avoiding high gain settings on that preamp. Must get it fixed though!
Cheers
Richard Howey
Audio Dynamite Ltd
IDR48/IDR16/T112/R72/Mixpad,Tweak,
Dual M-Dante/DVS, 17″MBP/Logic 9/Custom Mackie Control