TM751 has no powerline receiver and can not be controlled by a powerline controller only an RF remote and also will STEP on any other X10 signals on the powerline. It is hard wired to unit 1 and only the House Code can be set.
RR501 has an X10 powerline receiver and can be controlled by an RF remote or a powerline controller. Can be set to Unit 1 or 9 and since it has a powerline receiver is polite. If it sees a powerline transmission it holds off its message till the line is clear.