12-30-12: VladimirThanks, Vladimir. However, as I indicated in my first post dated today, based on my research it appears that these particular Krell amps are not differential or balanced or bridged. I suggested confirming that with Krell, though.
I checked the Gibraltar manual online and on page 12 it explains the way to connect to differential amplifiers such as your Krells, basically to connect the black Speakon wire to a chassis screw as was explained by others here.
Regards,
-- Al