have you considered buying a preamp with pre-out/main in.
ANY sub (self-powered), using line in/internal crossover/line (minus low bass) back to preamp is the best way
because
the self-powered sub’s amp takes the job of low bass, most of the power needs
the stereo amp driving the main speakers doesn’t need to make low bass, thus it can be lower power, less cost/size/weight, more placement options
and
main speakers don’t try and make low bass, thus they do a better job.
this is the best way to try tube amps with speakers of lower efficiency than horns, or other high efficiency ones.