the RCA preamp section of the amp is designed to accept an un-amplified signal.

If you are going to use the low level RCA input, you need to get a hi-lo adapter to reduce signal. This also solves your physical issue of converting a speaker wire into an RCA.

If this amp has a hi-level input, then it will have a separate connector or terminals that are designed to accept spkr wire, not an RCA.