With the adapter you give up all the benefits of the balanced line including noise rejection.
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Whether or not you have a problem will depend on the amount of noise present. If there isn't a lot of EMI in your environment then it won't be noticeable. If there is, you will have noise problems. If you have no choice and experience noise problems, try playing with how you have the cable laid out. The cable will act as a recieving antenna and the orientation of an antenna makes a difference in how well it picks up the EMI. Another solution is to use a balun, or isolation transformer, between the balanced line and the adapter, but do so right at the preamp.
The theory that a long cable needs to have XLR's is not true. When Sheffield lab recorded in Wiley Chapel, they used a RCA-RCA cable which was a coax cable for about 300ft. If you have any of the Harry James Recordings, you will have to listen real hard for noise, and the dynamics are something most recordings only dream of. The XLR makes for a better connection as RCA's vary in size enough to have loose fits and noise. If the cable is shielded and low capacitance, long runs are benign, provided the impedance is appropriate. many "balanced " designs" use the XLR and only hook up two of the cables, using the pin 1-gnd, pin 2+, pin 3-. If this works and is quiet and sounds good, enjoy the music and forget about the parts. the best thing to do is just remove the rca, run the cable inside, and solder it to the board, no connection is the best.