My reading of the specs (like most on here, I've not heard the Mackie) actually makes this less of a clear cut choice, on performance grounds, than you might expect.
The Mackie is nearly the size of a Monolith, but uses a passive radiator in place of the port employed in the Monolith, so they're both efficient large designs with a similar tuning point.
I think the Mackie could be a very good buy, but it ain't pretty and it is old without a warranty so that is a consideration. However, it is one of the few
PR/ported subs I've seen that uses servo feedback, so it should favour low distortion over output at all costs. This alone makes it really interesting to my eyes, but I am more than averagely risky when it comes to subwoofery.
How about you buy the Mackie and then lend it to me?
Russell