I think I understand your problem now. A friend of mine built me a neat little switch box (really a passive preamp) that has two pairs of switchable RCA inputs that route to one pair of RCA outputs. Also, each input has its own potentiometer so the volume of any input can be varied independently. With such a device, you could connect the receiver preamp outputs (sub out) to one (or one set) of inputs, and if the tube preamp has a second pair of variable outputs those could be connected to the other pair of inputs, while the primary outputs are connected to the power amps of the main speakers. The individual potentiometers would allow you to set the bass level for the receiver and the tube preamp separately so that after you initially set the levels all you need do is hit the selector switch to go from the HT to the 2-channel stereo mode. So I guess the question is -- does your tube preamp provide two variable outputs?