Hi Marco,
I had the same issue with my A10tx and CMC6 preamp and CMIT-5u as well, just too much RFI noise. All my Sennheiser boom microphones work great with the A10 system. Back to the Noise issue, I thought this was a broken (Blue Cap) cable out of the box, but it wasn't I pulled the next new (blue cap) cable out and observed the same problem, Noise. The noise does seem to fluctuate when moving the Blue Cap cable, from OK to bad to worse. I'm no electrical engineer, but I believe the issue is the cable itself. It might be too thin to shield a clean signal to the transmitter. Also, I already had one of these (blue Cap) cables die already on the lemo end ( I do not like these connectors).