Could this be a region specific issue? I'm in the UK so have the AC line frequency in the FM9 set to 50Hz.There is an unresolved issue with the FRs though. If you plug in a 1/4 jack for input it will have a low level hum. Balanced, unbalanced, whatever. It has a small ground loop hum using the 1/4 input. It will do this on a stock pre or a V2.
My FM9 is connected to the FR-10 from Out 1 (L) with a single , standard 1/4 jack cable (not even a humbuster) and I'm not hearing any hum, even with the FR-10 volume maxed.
It is, of course, quite possible that my hearing is shot!