As Haggs said, -2 is the lowest value for voltage offset. If you look at the code Haggs posted it gives the accepted range each switch will use.

To drop lower you have to use the --Avalon7-voltage switch in Other Options. I went as low as 10 at which point the 721 would only start enough to run at 1.5THs. Using voltage 11 fired up all strings but put speed around 3.5THs. 12 with -2 as offset was the sweet spot for me with very low power use (and temps) and decent hash speed.

edit: for voltage adjustment the command code is
 --avalon7-voltage-level 
 "Set Avalon7 default level of core voltage, range:[0, 15], step: 1")

You can see the result of different numbers by looking at the cgminer API log for the string Vin and Vout