Shortly after getting my S2000, I decided to upgrade the stereo system by installing better speakers and an amplifier. I had a set of Bass Shakers left over from a previous car, and figured in conjunction with some decent door speakers and an amplifier there would be a noticeable improvement, even with keeping the stock head unit.
Since I'm not a half-deaf bass-a-holic with a desire to suck the life out of the alternator, I bought a modest 50W x 4 amplifier from my local Kenwood factory outlet. It bench tested at a reasonable 62 watts per channel (rms) with 13 volts input. I installed it above the gas tank, trying to conserve as much trunk space as possible.
The speakers came from my long-time speaker source, Madisound (.com). I've been buying from them long before they had a web site, even before Al Gore invented the Internet. The woofers are Audax Aerogel Cone 6.5" drivers (model# HT170Z0 Fs 50 hz, Qts 0.56, Vas 10.9 liters, Xmax 3.5 mm, 60W, 87db). I chose a Vifa textile dome neodymium magnet tweeter to go with them ( model# D26NC47-06, 6 ohm, 30 watt, 88.5 dB, Fs 1800 Hz). Since the tweeter was 6 ohm, I didn't worry about it being slightly more efficient than the woofer. These guys were about $200 with shipping and cross-overs.
I used 2 channels for the door speakers, and two for the bass shakers, which I mounted under the seats, where they actually make some audible bass since they are mounted on the floor, not on the seats.
The resulting system sounded so good I put off installing a real subwoofer for almost 2 years. Some listeners thought I had an 8" sub hidden somewhere, and were surprised to find I didn't. Here are some photos: