So you set it to 9.4v for every frequency. There really is no underclocking then since when you set the voltage it sets that for each of the 21 lanes. 9.4/21=0.44761V a chip. If you are trying to underclock it for better efficiency you would need to recompile bminer and lower the voltage to whatever works best for whatever frequency your using. This is only good if you are trying to get the fastest th/s and don't care about losing efficiency since autotune was made for increased efficiency.
Fixed freq FW adjust voltage depending on frequency. Higher freq means high quality of chips, so FW will set 8.8V per chain. If you use 600MHz, it will set 9.3V for the chain, because it expect lower quality of chips. I compiled binaries which do not respect quality of chips and up to 675MHz will set declared voltage (eg 8.4V). 675MHz and more will force voltage to be at 9.4V for easy OC without copying new binary.
So, for underclocking, copy bmminer with 840mV (8.4V) to your /usr/bin directory. It will force 840mV for all frequencies up to 675MHz.