The problem is: RCA is unbalanced signal and the speaker wants a balanced XLR signal.
So even though you have an adapter it does not suddenly make your signal balanced… and you get all the nice distortions from the outside world (in case of hum I expect the power connectors).
So long story short you need something that will make your signal balanced.
Like this thing there:
http://www.parts-express.com/rolls-mb15b-promatch-balanced-unbalanced-converter-35mm-rca-xlr–245-1062
Why RCA does work in many clubs is, that you plug in your RCA to RCA cable into their mixer with a very short cable and the PA output of that mixer is a balanced XRL or TRS cable.
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