that sub-woofer only has line level in as others have noted.
IF you change to a different sub, some subs, like my Velodyne Servo 1200, have the option of speaker wire inputs or line level in. Adjustable crossover and Volume.
In your setup, the 2 pairs of 901’s always get full range signal via the selector box, and the Velodyne Sub (or similar type) would receive full frequency signal from your speaker selector, then sub use’s it’s adjustable crossover and it’s volume control for only the bass frequencies that you select on it’s crossover. It has speaker wires out, that normally/optionally go to the satellite front l/r, in your case you would not use them.
Another option would then exist:
1. selector one: full range speaker wires to a pair of 901's
2. selector two: full range speaker wires to sub for extended bass
3. speaker wires (after crossover, thus no low bass) from sub to other pair of 901's
selector 1, one pair full range 901's only
selector 2 sub for extended bass, other pair of 901's (now they don't try to produce low bass).
selectors 1 and 2: add the above.