I had use a Niles SAS-1 as a switcher to select output between a AVR sub-out and a active low pass xover from a two channel preamp to a subwoofer via RCA cable without any problem. Actually, any line level source switcher/selector can get the job done, and I believe if both source are turned on will not causing any problem as long as only one source is selected and connect to the sub input.
The only possible problem I can think of is most of the RCA switcher/selector only switching "+" side and the ground connection remain connected. In some cases, it will introduce ground loop hum. The solution is modify a xlr switcher to rca so both "+" and ground connection can be switched on or off.
Another possible problem is "cross-talk", it can be overcome by simply turn off the unwanted source.
The only possible problem I can think of is most of the RCA switcher/selector only switching "+" side and the ground connection remain connected. In some cases, it will introduce ground loop hum. The solution is modify a xlr switcher to rca so both "+" and ground connection can be switched on or off.
Another possible problem is "cross-talk", it can be overcome by simply turn off the unwanted source.