Well, I guess my request to Moth3r to delete this thread has been denied.
In a nutshell, the problem was actually the preamp mixer, and I didn't catch it because I was trying to do something else at the same time. That won't happen again.
The new preamp mixer will be here in roughly two weeks for me to give it a go again. If that doesn't solve the problem, the project will definitely be over as I will return everything to the manufacturer and be back to square one.
So, fingers crossed....