Sending power to passive subwoofer via RCA connections
You need to see what size of wire was used for that subwoofer connection.  If it is small-gauge audio cable, that pretty much rules it out for carrying any significant power levels.

Generally speaking, an RCA connector is good several Amps of current.  I quite like that connector - it is robust and reliable in long-term use.  RCA connectors are often used as speaker connectors in consumer applications.

But the interconnect wire is quite something different.  If you are lucky, the wiring was installed by a professional who used shielded twisted pair #22 AWG or so.  If you aren't lucky, someone went to a local consumer store and purchased a cheap RCA extension cord - these can have conductor size as small as #28 AWG - with a really crappy shield.

15 Feet of #22 wire is good for tens of Watts.

But: the cable from the amplifier to your subwoofer should be much larger than those.  I would normally install either #14 AWG or even #10 AWG, depending on how much power you plan to feed to the speaker.