C A R A U D I O
JBL produce brilliant automotive and home / club speakers, some of the best of which are their subwoofers. These aren't the top of the range GTi model, and don't boast a thousand watts as the GTi's do. The are rated at 125 watts RMS and currently get wired to a 2 x 200 watt RMS amp, which has the gain turned down. I currently have the subwoofers mounted in an isobarik box in the back of the BMW being powered by the Annihilator HA400C amplifier, one of which is wired out-of-phase to produce a push-pull effect.
The subwoofers are fed only a mono signal from the Phoenix Gold crossover to the amplifier which powers the sub. This is a must as minor differences in the left and right channels of the sound would reduce the sub's sound quality and possible damage them. The Annihilator could be bridged for the subwoofers and they could have been wired in series, but as this type of isobarik box needs one speaker wired out of phase, both channels of the amp are used, one for each speaker. The t/s parameters used to design the box are in the table below but as the speakers were both bought second hand, they're isn't a lot of manufacturers information about them.