Subwoofer issues


I have BOB CARVER'S Sunfire True Subwoofer Mk II, and when I connect it to my tube preamp, the gain reduces by about 60% so if Off is 6:00, comfortable volume is 9:00, but when the sub is connected, the volume must be increased to about 12:00 to have the same level as before.
Would anyone know where the problem is or how to fix it
Thanks all for your input.
Regards
Convert?fit=crop&h=128&policy=eyjlehbpcnkioje0otu2otg2mdesimnhbgwiolsicmvhzcisimnvbnzlcnqixx0%3d&rotate=exif&signature=6ee61a38fb168cb656bc7f619cd3ddd1ad2ac227cc6517947861b99d1ca9a284&w=128marcel1728