This is a niche product that will probably find few homes. It is what it looks like; a small speaker in a drum shell on a fancy stand. It's called a "Sub-Kick," but if "sub" means subsonic, you'll be disappointed. I'm not sure why, but this implementation of an old recording engineer's trick is severely lacking in low end output. I haven't disassembled the unit, but maybe the problem is in an output impedance-matching transformer (if Yamaha used one)? Maybe the problem is that the acoustic character of their drum shell is such that it limits the 8" speaker from moving at very low frequencies? Maybe Yamaha doesn't know why engineers resort to this technique in adding lows to a kick drum sound? Maybe, because of tools like Shure's Beta52, EV's N/D868, or Audix's D6, the reason for a big LF driver has vanished into history? I don't know the answer, but I know I can't find a practical reason to add the Sub-Kick to my signal chain.
The output of this device is heavy in "whoomph" and light in "thud." Technically-speaking, the majority component of this device appears to be in the 80-150Hz territory, which isn't low enough to create a big sounding kick or high enough to add impact to the sound. It appears to be more of a funny-looking effect with little practical application.
To get anything useful from the Sub-Kick, you have to get it close to the head and beater. In the picture above, the Sub-Kick is shown several inches from the back head, which will result in a thin, muffled sound that would be completely useless when combined with the large element dynamic mic they are showing in the back head port. If that same combination moved the Sub-Kick as close as possible to the head, something more useful can result. Maybe. If Yamaha's fancy drum stand mount included enough hardware to get the Sub-Kick inside the drum, really near the beater head, something resembling sub-kick information can be obtained from the Sub-Kick. However, that requires you to re-engineer the rig and it's a lot easier to simply put a small speaker in the kick drum, on a pillow or blanket, and save yourself some money.