You don't have to get 2 ohm components so that your sub channel can run at a 2 ohm mono load.

Amplifiers react to the load they are presented with. Remember, the speaker's impedance varries greatly across the audible spectrum, it's never going to be exactly it's nominal impedance.

Post a bit more info about your plans for your system, as there may be a better solution out there for you then just a 5-channel amp.