What I have done with my tube amp (Ayon Spirit II) is utilize the pre-out and send that signal to a second integrated amplifier through which I power a second pair of speakers. My tube amp powers the 4 ohm speakers (Montana EPS2's) and a McIntosh integrated powers my back speakers (8 ohm Vandersteen 2a's). This prevents entirely the problems that Almarg mentioned above. Further, with two integrated's I can taylor the front vs. back speakers beautifully. Running two channel 4 speakers makes the system sound DRAMATICALLY larger, more full, and realistic with vastly superior sound stage. The difference between the 2channl/4 speaker and the 2 channel/2 speaker is staggering (akin to tripling system cost or more)
So... that is what you should consider doing. Of course, your tube integrated needs to have a pre-out to do it...
So... that is what you should consider doing. Of course, your tube integrated needs to have a pre-out to do it...