Powered subwoofer impedance

Many thanks for your help, Erik!

Regards,  Mark

So long as we are talking about a powered sub, uou can ignore the subwoofer.  Pretend it isn't there.

There IS an impedance of the input of the subwoofer circuit, but it's usually in the 10s of kiloohms, so immaterial to the overal value. The amp might see a difference of 0.0001 ohms at the end of the day.

self-powered sub's impedances match their internal amps which are designed together 

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.