I assume you were probably using a balanced 3-way splitter. If the signal was too weak for the modem with a balanced 3-way splitter, then you could try an unbalanced 3-way splitter and have the modem running off the stronger output.
Looking at your diagram, if you are having weak signal problems already for the cable modem, then you should remove the 2-way splitter before the MoCA adapter and the cable modem. There really isn't a need for it. Just use the RF output on the Actiontech adapter to pass-through the coax to the cable modem. This is actually the setup that the TiVo website shows and is how I have my network setup, and it works just fine for me. As BigJimOutlaw points out below, you can install a POE filter between the adapter and the modem if the MoCA signals happen to cause any problems with the modem.
Also, your diagram with the diplexers may be more complicated than it really needs to be. I have read other people on here say that having MoCA going through a powered amp can cause problems, but I have mine going through one and have never had any problems with it. I'm sure it probably depends on a number of factors such as the specific amp you are using, but I'd try sending the MoCA signals through the amp before resorting to a more complicated solution that might not even be needed.
How many other TVs are in this house besides the one hooked up to the Roamio and the Mini? Is connecting the Roamio to the one Mini the only thing that the MoCA network will be used for? Are the other TVs using digital cable boxes or analog cable without a box?
Last edited by tarheelblue32; 03-30-2014 at 12:49 AM.