First of all, the cable you used is the wrong one. You are trying to fit L+R into one balanced signal. So instead of the speaker receiving + and – on tip and second ring of the TRS, it’s getting +L on tip and +R on ring (or other way around).
This will not work. Although running unbalanced (RCA) to balanced (XLR or TRS) is not a good idea if you have any serious distance to cover. The cable is highly susceptible to outside interference (hiss, humm and a variety of other issues) and also the RCA output level is significantly lower than the level needed for TRS. Meaning you will have to run your master a lot hotter. That need not be a practical problem, but it is not as it should be in a good setup.
Is there a solution? Yes, there is.
Get an RCA-RCA cable (single preferably) and an adapter like this one: http://www.scan.co.uk/products/stagg-ac-pmcfh-audio-adaptors-rca-phono-(female)-to-63mm-trs-stereo-jack-(male). Use on of the RCA channels and attach your RCA cable. Then run the other end into the speaker. Your speaker should now play the sound for that channel (L or R depending on what you hooked up). Hook up the other side in a similar fashion to the other RCA and speaker and off you go.
If, and this is possible, you are experiencing interference of some kind and/or signal level issues, you might consider something like this: http://artproaudio.com/artcessories/audio_solutions/product/cleanbox_pro.
You run your short RCA stereo cable into the inputs of this box and run XLR or TRS balanced cables out to your speakers. Finished!
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