To reiterate, adding a sub can make or break an audio system.
Mostly, when people are looking to buy a sub to add to their rig for the first time, what they are basically looking for is more "viscerality" in their music/movies/media.
Adding a cheap sub is the easiest way to do this but it has its caveats. Not only are cheaper subs blatantly inaccurate, they often lead to unnecessary bass loading or boominess in the sound. Such subs provide you low end oomph at the expense of quality and what you end up with is mashed potatoes instead of french fries.
However, the biggest problem in using such subs is their inability to synchronise or "melt" into the system. Anyone with a critical ear can easily make out something vibrating in the corner of the room (or wherever you have placed the sub). This effect is generally compounded when such subs are used in a stereo only system.
My personal advise would be to go for a good sub rather than a cheap one. Unfortunately good subs aren't cheap (none that I know of at least). The sub in question looks nice but I would doubt what a "german" manufacturer can give you for 15k while having their subs made in china. I would take all the positive reviews with a pinch of salt before auditioning and more importantly buying; no matter how lucrative the price.
Unless the system is mostly (>80%) used for movies; I would suggest the OP to save the money for a upgrading to a better AVR/ Invest in a quality DAC or transport with multiple inputs / better interconnects , etc; rather than spending in a cheap sub. One could also try repositioning the speakers to get bass support from the walls: very underrated fix if you ask me.
Hope this helps!
