I loved my Killer Kaos. That was the one with the Rossi .67 I agree on the prop. Never liked MA. I mostly fly gas now and use Xoar almost exclusively. For my smaller engines I like APC. The bigger MA props flex so much, they are considered paint stirrers! I have had very good luck with the Jett Mufflers. You have to prop to them though. They have to be able to come on to the pipe. Generally I have found you have to down prop to get the RPM up high enough to come onto the pipe. That may be an 11-6, 11-7 for a .61