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