A test done in another comparison post here has indicated that approximate runtime with phantom on and mics that draw 10mA current can total at least 2-4 hours with the 2700mA Ni-MH AA's and supplied lithium power.
Can you safely or efficiently use various Lithium AA batteries in this unit? From past experience, these far exceeded the runtime capabilities of Ni-MH AA's in my old D100 DAT unit and was reported in about all other applications. If it were possible in this unit, I would assume that when setting up you would cheat and select "Alkaline" for the choice of AA since there is no "lithium", and the batteries would do the rest.
This all said, If I started with two good pair of lithium AA's on me I could switch back and forth from the unit's lithium (low but not dead) battery while switching out the previous set of dead lithium AA's, which should theoretically tape any conventional length evening event without a break. Now if this all is indeed true, I see nothing wrong with this unit and potential power issues!