i have a Pioneer SX-727 Stereo Receiver that I used the Mic Jack on the front panel to run a bluetooth transmitter through via an 1/8" to 1/4" headphone jack adaptor, and I get really good sounding audio that is stereo through that method.
If you have a bluetooth transmitter that has a rechargeable battery in it to power it (you charge it via a micro-USB jack) then you really don't need an isolation transformer, just a Stereo RCA jack to stereo 1/8" headphone jack adaptor, which you can take and combine the stereo RCA signal into a mono RCA signal by using a Stereo RCA Jack to a single Mono RCA Jack adaptor, and I've done that before on several of my old tube radios that had a single Mono RCA Phono Jack on the back of the radio and the audio quality was quite decent.
