I totally agree with Bob's excellent response.
"I would like to know how they perform with rock music and if any deficiencies would be cured by pairing a sub."
Yes, the deficiencies would be cured by pairing a sub, but only with a high pass crossover as Bob described. Your Rel T5 has a low pass crossover for its input signal, but this has no effect on the main speakers. They would still be playing full range. Also, with the Peachtree Nova 125se, there is no way to use a line level (RCA connection) high pass crossover, separate box or built into a different sub. With the current equipment, your only option would be a speaker level high pass crossover.
Keep in mind that this is not just an issue with the LS50, but any small speaker.