to compare speakers from one amp, or switch which speakers are playing: I used an older version of this Niles unit, individual volume control for each set of speakers
https://www.crutchfield.com/p_190SSVC4/Niles-SSVC-4.html
smaller 2 speaker set unit
https://www.crutchfield.com/p_190SSVC2/Niles-SSVC-2.html