Old question that was never answered but if you are still for a Mac clipper looking you might wanna check Airwindows' Crystal.
Oh no no. trust me, I AM Chris J of airwindows, not just a noob metal guitar player putting up mixes
No, the one you want is 'ADClip'- it's way better at the job than Crystal (mind you, Crystal is free...) But more than that, what you want is an approach- I started off mastering before I got frustrated at the loudness wars and stopped trying to get gigs (ironically, just around when I finally kinda knew what I was doing)
The approach I like is 'slow to fast'. You use compression, then limiting, then clipping- each one contributing only part of the loudness gain. Since I get to use my own stuff freely I go for 'VariMu' into a gain boost of any sort into 'ADClip'- since I also put out a hell of a lot of free stuff you can play with the basic technique running Pressure (like VariMu but more colorful) into say Drive (or if you're OK with softer saturation, Density, which is a very handy and very basic tool)
http://www.jinxtigr.com/f/Pressure.dmg
http://www.jinxtigr.com/f/Drive.dmg
http://www.jinxtigr.com/f/Density.dmg
and here's Crystal in case you prefer that- I will admit it gives you more control over soft vs. hard clipping (but people weren't as into the sound of that algorithm)
http://www.jinxtigr.com/f/Crystal.dmg
and here's a bass SVT cab plugin in case you need that kind of thing on hand
http://www.jinxtigr.com/f/CabSVT.dmg
I think I do pretty well at getting things loud, but more than loud I want loud and BIG and full-range, so that always makes it a lot more challenging