I have to concur with duck69's 1,2,4 and especially 5.
Know your products and it's strengths and weaknesses. The TM751 will only give you problems if you go beyond a basic setup. If it's the only transceiver on your powerline it will do it's job and do it well.
If you put multiple on the powerline it will still do well, given that you keep them on seperate house codes, the odds of them firing at the exact same moment is still there but manageable.
What about X10 statuses from Modules? The TM751 will likely collide with those messages, but if you are getting statuses from modules, you would have upgraded to a computer controlled interface or a device that is transmitting status requests and you'd better do away with the TM751.
My two cents is in response with mackie1064's original question:
"Rather than doing the Dryer thing and/or the phase coupler stuff......can I just add a couple TM751's? If not, what about the RR501's?"
Keep the phases seperate logically and let each TM751 manage that phase.
"I would think just adding a couple of these around the house would fix my phase issues..but maybe having multiple will send the signals twice...(I thought adding the RR501's to the TM751 would fix this)"
Adding more TM751s on the same phase will worsen and not help your outlet issue. Sending the signal twice does not make the signal stronger. Sending them at the same time actually makes the signal harder to be understood.
My general interpretation of the question is that there's an outlet not getting a signal, if I put a TM751 next to that outlet (ie on the same phase) will it help.
Yes it will help turn that module on. But if it's on the same HouseCode as the other TM751, it likely fire at the same time and will cause signal collision. A phase difference does not gaurantee that no signals crossing from one phase to another.
If that's what you want to do, that's the weaknesses you will need to deal with, but you can deal with them. A good X10 setup does not requre multiple TM751. Boosterlinc / SignalLinc are design to help carry that signal throughout the rest of your large multi phased house.
In general advice #3 will save you from a lot of headache in the future if you want a good X10 system. But I would not go as far as "do not use" them, but understand their limitations.