It is better if your sub is self-powered with line-in, a built-in adjustable crossover, and line out back to the primary preamp/amp
to do this, you need a preamp or integrated amp with pre-out and main in. you go line level out to the sub, it takes the low bass, then it sends upper bass and higher (high pass filter) back to your amp's main in.
this way, your new amp will not need to or try and make low bass, and, the bose or any future replacement mains also will not try and make low bass.
this is the best way to reduce the power needs of your amp: smaller cost/size/weight/heat, greater options for location which may effect cable lengths and infrared remote beam.
thus, the best way to try tubes, especially if speakers are efficient.