1. Do you have the PAD button engaged for your keyboard/synth inputs? You need to have that engaged for those inputs per the manual.
2. Do you have the latest driver for the midex? There is a new one available BTW
3. Try just using ONE MR816 and unplug the midex, see what happens.
4. You have the SAME issue with 2 diff MB's. I would re-check the power supply for the room.
5. YOu have the same video card in both boxes and he same P/S. Could be either one
6. Your using Cubase 5.5.3 yes??
7. What HD's you using in these boxes?
8. Windows version?
9. Both boxes have the same VIDEO and P/S , something to check or change to see
10. What are you using for the internal soundcard? This could be the issue giving you noise
That said, the only common thing is the MR816X itself, you may have a couple of bad units.
Just one bit short of a byte