Recorders with four-channel bridge amplifier
For four-channel bridge amplifier, which is equipped with almost all modern audio players, these options enable the AU can be combined in different ways. For example, using both "mix mono" and the usual connection diagram (Fig.5), it is possible to connect a subwoofer and tweeters or rear speakers (with limited bandwidth), and the remaining channels will be used for mid bass or front speakers. 
Fig. 5
Another option is the three-way front speakers with a combined connecting and miking scheme Heffler. The fader allows you to adjust the ratio of MF and LF, the level of RF in the regulatory process varies 6 dB. The procedure for connection to the load selected such that overloading of the amplifier does not occur even when using the bass heads in a full band of frequencies and midrange only with HPF, which makes this scheme particularly attractive - no need to wind inductors.
Fig. 6
Recommendations
Because you are using inverting and noninverting outputs built-in amplifier, pay attention to the polarity of the heads. In cases where the scheme requires the connection of one of the load leads to "weight", this should be done on the body of the radio or on the "minus" the power wire as close to the body. The connection with "weight" of the car is unacceptable!
For schemes with a bridge connection option loads in the composition filters you want to use non-polar oxide (electrolytic) capacitors. In extreme cases they can be composed of two polar twice as much capacity, connecting them counter-sequentially ("plus" to "plus"), but the sound quality with the non-polar capacitors above. A capacitor of small capacity it is better to use paper or polypropylene.
The calculation of the low pass filter for the subwoofer is made in the usual way. However, the design of a filter of the second order, it is desirable to introduce a resistor of 1 Ohm, as shown in the diagrams. This will reduce the likelihood of overloading the amplifier of the radio.
The frequency partition and order LPF for subwoofer are selected within 80...200 Hz depending on the location of low-frequency drivers. If it is placed in the rear compartment, the frequency partition must be selected as low as possible and the procedure above to avoid playback subwoofer "voice" range. However, this requires the manufacture of a truly cyclopean inductors. On the one hand, to use in their design ferromagnetic cores is undesirable because of the distortions caused by the unavoidable magnetization of the core, greatly diminish the sound quality. On the other hand, the use of the core allows to drastically reduce the resistance of the coil and the loss of it, which is especially important when using built-in amplifiers audio players. So the choice should be made in each case. In short, the options are many - would be a fantasy.
Author: A. Chechetov; Publication: www.bluesmobile.com/shikhman, www.cxem.net