Wayne Parham Messages: 18835 Registered: January 2001
Illuminati (33rd Degree)
Those square devices are 0.1uF capacitors. They look like polyester film parts, and if so, you want to replace them with polypropylene capacitors. I'd probably choose Panasonic or some other brand that I trust. See the link below for information about capacitors:
Beyond that, since it's purely passive there will be some insertion loss. Generally we need more gain for the subs, because they are usually about 10dB less sensitive than the mains. This device will put us probably anywhere from 3dB to 20dB below that. So make sure you can dial down the mains to compensate.