HH Scott 350 fm tuner

Hi,
I just bought a Stereomaster 350 tuner in good working condition. I went to someones house to pick it up and he had it playing through his desktop sized speakers but that was enough to decide that it was worth the $100.00 price tag.
The problem that I had in setting it up is that my cable rca males are too large for the narrow and close together rca female inputs on the back of the 350 chassis. I was able to partially plug in the monster cable ends but I'm getting very low volumes so I had to turn up the volume on my ASR Emitter II all the way just to hear anything. I'd rather use the Monster Cable than a $3.00 cable from Walmart.
I'm wondering what other people have done to remedy this problem. My initial thought is to find extensions that will fit on the end of the Monster Cable but will also plug into the Stereomaster however my question being, which extensions and where do I find them?
Thanks!