I'm looking for a free mastering solution ... the paid plugin I use under Windows is not available for the Mac.
Jamin is a really nice one which I first encountered under Linux using Jack. I have JackOSX installed so I can use SuperLooper.
There's a (probably out of date) long set of instructions I found in the Ardour forums explaining how to build it from source, but the list of dependencies is kind of daunting, so I don't want to attempt that if someone has already done it.