Software crossover
This is proposal to implement a software crossover in VLC media player. Software crossover makes it possible to achieve multi-amplification which can greatly improve sound quality. All details below.
Crossover is a device that separates audio signal into frequency bands that are reproduced with different loudspeaker drivers. (What a crossover is). Any commercial 2-way or 3-way loudspeaker contains a passive crossover network inside that does this separation. (In its simplest form just a couple of coils and capacitors). It is done this way because those loudspeakers have to be compatible with common components (amplifiers). Passive crossover network causes phase problems, and it wastes power and has other problems (more load on amplifier, etc).
Multi-amplification is using an active crossover prior to the amplifier, and then amplifying the signal with several stereo amplifiers (or with one multi-channel amplifier). This is a great way to achieve better sound quality, but you have to hach your speakers to disconnect their internal crossover network. Some information about multiamplification: 1, 2, 3, 4.
OK. Basically the best way is to implement a crossover as close to the signal source as possible. Ideally even before D/A conversion. And now this is possible when a computer is used as a source, using a multi-channel sound card. One working example of software crossover is a foo_dsp_xover plugin for foobar2000 player. It allows you to select filter type, order, crossover point, and it supports up to 8 output channels in total (4 for each input channel because it is just stereo). Another working example is channel divider (also plugin for foobar2000).
Unfortunetaly I am not aware of any video player with crossover functionality. So, while I am able to enjoy music using computer as source and multi-amplified speaker system, I can't watch movies this way. It would be really great to implement the software crossover in VLC player.
I realise that this is not a trivial task. Proper crossover is more complex than duplicating a channel and applying simple equalization. The shape of the frequency curve is very important. One possibility could be to cooperate with the authors of other existing freeware software crossovers, like the two plugins mentioned above. Another possibility could be to implement interface to foobar DSP plugins in VLC media player, but that would work under Windows only.