I would prefer to turn the RCA signal in to a mono signal as I have a mono amp to run the speaker. If I cant find some way to do that then I guess I will find a vintage receiver with the mono switch.
Thanks
You could use any type of cheap little mixer. What's your budget? I was doing something like that with a Peavey mixer, and I think it was about $80. If you're interested, I might be interested in selling it; I was using it exactly as you would to solve your problem and it worked fine.
Or, if you have need for parametric EQ, you could get a Behringer DCX2496, use two inputs for the issue you're trying to solve here, and the other remaining input to EQ your sub.