I just did a bunch of 4CM testing with a SSS Strat, a RedPlate Blues Machine 66, and the FM9 using the factory 4CM Template preset. I turned the noise gate off in the Input 1 block of the FM9, left all the effects bypassed. The idea was to see how much noise I got with the unit hooked up vs. plugged straight into the amp. The amp was turned up about half way (50 watt amp), and set fairly clean with a little "hair" (I'm not a high gain player) so it was pretty loud. I am using humbuster cables.
Honestly, I really can't say that there was any more noise or hiss with the FM9 in 4CM than there was just plugging straight in. There was a difference in the volume levels, the FM9 was pushing the FX return to the power amp harder and it was louder in 4CM vs. going straight in without anything in the loop. I could have turned down output 3 to compensate, I had it all the way up, but since the noise level was about the same as going straight in I didn't bother.
I did just check and I see that there is also a noise gate assigned to the Input 3 block. I did not turn that off (didn't think to look) and I'm not hooking it up again to test that. So, I'm sure there would be a little more noise had I turned that off.
I don't normally use 4CM so this was just a test. I do have to say that this is the first time I have tried the FM9 in 4CM since the last update to the Drive block. I really liked what I was getting from the Drive block in front of the amp.
Anyway, that that for what it's worth. I would be satisfied using the FM9 with my amp based on this test.