Having tried Speakercraft Boomtomb and the Niles garden subwoofer, I can claim to be an expert on garden subwoofer as I had to replace each of those units twice and even that wasn't enough. For those brands, even though they rate them as 500W capacity, they most certainly are not able to reach even half the stated capacity.
With this Martin-Logan, I was careful to inspect the unit thoroughly and I was very surprised at some of the things I saw. Instead of a flimsy plastic structure that is used to mount most of these subwoofers, the ML used a one inch thick hollowed out round ring structure that provides a completely stable mounting super structure.
Why is this important? One of the chief downfalls of outdoor subwoofers is that the mounting structures themselves are sufficient to keep distortion levels down so many times the bass you are hearing includes the actual vibration occurring within the shell itself. That means that your frequencies are going to be dampened or distorted by these vibrations. It's part of the reason that advertising that you're making the ground move is actually a horrible gimmick.
The idea behind the in-ground sub woofer is to force as much sound up through the outlet rather than allowing the frequencies to pound on the container. Although, the aluminum mounting structure doesn't eliminate these issues completely, it does do it enough to actually matter.
The second aspect that ML has over most of the competition is that most of them for some reason have set up their sound paths to involve some sort of deflection. In other words the sound you are hearing is actually the sound as it bounces off a deflection board so that the sound you are getting is actually after it has been dampened by some obstruction.
For the ML, again getting back to the mounting method, the speaker actually is pointed right straight at the exiting tube. This means that even through you are still have the potential of some dampening of the upper part of the box, most of the sound is traveling right directly through the exit tube.
Third, the exit tube's length is minimal, meaning that there is less travel time for the sound to exit which in turn means cleaner and clearer sound. The tube itself is topped off with a mushroom-shaped top. The topper is not cheap plastic either; it is actually a weather-resistant painted metal top meaning it could very well have been heat dried for longevity.
As for the box itself, it too is minimal, allowing just enough room to fit the aluminum mounting super structure with only a few inches of clearance to the top. This is another thoughtful design that emphasizes the priority of getting the sound out quickly and cleanly.
As for the actual box itself, it isn't cheap PVC or the like but rather feels like a composite almost. Whatever the process, this is no cheap plastic container but rather a very rigid container.
But what about the sound? Currently, I recommend 100 hours of low to medium volume. Do not crank it up until the break-in period is complete. After 50-60 hours I did test at higher volumes for brief periods of time. I have to say, the sound quality is actually quite good with little or no distortion. The true test of any sub woofer is Pink Floyd's "Us and Them" and this unit performed wonderfully.
I am running it off a Niles amplifier rated to 100W in bridge mono mode because this is an entirely passive sub woofer which I would recommend.and the sub is rated to 800 meaning that there should be little chance of incident with those margins but even then the other brands cannot handle what this unit can
So you have a choice, Speakercraft for 600 or so, Niles for 800 and have them blow up or you can save your money for the Martin Logan. If you are serious about outdoor sound then the ML is the only way to go.