If the EQ should be set flat, why bother having an EQ at all?
When setting up a system to 'blend' properly, adjust all your gains to the maximum safe level (see gain setting tutorial), then adjust gains of the loudest speakers DOWN to match the output of the quietest speakers. This will blend the speakers together, for any volume level you play the system at.
If the subs seem to to dominate the front speakers more at higher volume levels than at lower ones, you may have some nasty vibrations that get much worse at higher volume levels, there by increasing output even more at certain freqs. Or, your front speakers may be hitting their compression limits given the amplifier power available to them. A larger amp with more 'headroom' for transient peaks may be in order, or simply better speakers with more power handling (and a bigger amp) may be necessary. Try turning your subs down to match the front speakers first though, before you decide you need to buy anything.