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.
How are you connecting? Rca line output from preamp to line input to sub? Has the hookup worked before and now doesn't.? Also, what is the make of your preamp?