The two additional pairs of speakers would not "only respond to the Krell's volume control," they would respond to both volume controls. If I understand your question correctly, I would think you would set the volume control on the Krell to where you normally set it for the main speakers, and then set the other volume control to provide the volume you want on the other speakers. If you subsequently were to change the volume setting on the Krell, it would affect all speakers similarly.
Regards,
-- Al
Regards,
-- Al