the KB45 8XL has tonnes of power... I've never used the big maxx before, but compared to my LMT 1950s... the KB-45 has more power... Of course, there's more heat build up due to the lower efficiency of the KB45.... But a fan does help quite a bit... it's awesome on 6S... but 8S is on 8S is just sick... but they pull some amps...
Most powerful, but less amps...
6S KB 45, 8XL may be much for that 9920.. you might need something a tad bigger, but depends on ur gearing too..