I would think that using a 9 volt batteries or any other smaller batteries wouldn't have the MhA needed to run the Sovereign very long, like using our 12 volt Sovereign batteries to run our cars.
It has a lot to do with the current needed to run our device and those smaller batteries are not made to run such higher current devices and if you ever do, like some of the older digital camera you notice if you took 10-15 pictures in a row the batteries got very hot.
The factory battery packs for the XS2 and XS2a used 10 of the AAA packs and were around 700MhA and back then most would buy the Sun Ray AA 1600 Mha as they lasted twice as long as the factory ones did. I remember one guy even hooked up a bigger 12 volt battery he hip mounted and run the wire to the control box and went for days without recharging as it could hold so much more and could be charged at a higher rate. If I remember right a 9 volt battery is around 70 Mha while our batteries we used in our GT are around 2000Mha.
Rick