You didn't ask, but I'll give you a few hints.
Speakers with passive filters do a creditable job of cutting high frequencies which make long-term listening tiresome. However, you can do just about the same thing by mounting the speaker under your seat and aiming it up. The cushion material will absorb most of the high frequencies, and accentuate the lows.
Any speaker smaller than 3 inches isn't worth the effort, nor are those which are 8 to 12 ohms. That's almost every after-market unit currently available.
The GE and Motorola speakers are much better. They're 4 inch, have large ceramic magnets, a 3.2 ohm impedance, and an SPL greater than 98 dB. Drive them with today's under powered (audio) transceivers, and you can actually get too much volume, even when mounted under the seat.