Hi,
Two perspectives. One, is the product itself. The other is the support & service. Personally I'm not going to buy a high-dollar item from a company that I cannot get reliable long term service and support from. So I look into this stuff much deeper before committing to a purchase. For this reason, I'm very hesitant with Monolith subs; for example, if your amp dies, what's your warranty, how does the claim go, do you have to ship anything and if so where does it have to go and who pays for that, and if you are out of warranty and need a new amp plate, are they going to sell you one in 8~10 years from now?
Don't compare generously over-estimated power or numbers from most advertisements. Just look for the frequency response graph and then look up reviews to see if its an honest value or not. Physics is at play, so a small cabinet with a little woofer is not going to do what a huge cabinet with the same size or bigger woofer will do. And the quality and power of the amplifier involved matters a lot. You can find these fairly large sized cabinets and big woofers with these way under-powered amp plates bolted on the back for cheap, for a reason.
Start with what your goal is with your sub. Low end extension for movies? Or speed for music? Then look up what kind of sub you're more likely to need for that or for both. Then look at what your budget is, and whatever it is, if it's under $400 I would argue to just start only looking at the $400+ range stuff and more probably look at the $500~800 stuff if you want a very good, competitive sub. Then start looking at what you like the look of and research if that company has service and support you can actually use.
Very best,