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:

New speaker left, original on right   New speaker left, original on right. 
Notice larger magnet and increased
depth of replacement speaker.
 
 

 

    
Without this 3/4" spacer ring the speaker
magnet would hit the widow glass.
Note serious crossover mounted on door.		   Inside of door panel, shows tweeter mounted next to woofer, wires glued down to inside of
door panel to keep them from buzzing.
 
 

 

    
Outside of door panel, after tweeter installed
Metal grill protects tweeter and almost looks
 like a factory system.		   Kenwood 4 x 50 Watt amplifier mounted
above the gas tank.  It barely fits between
the tank and the rear shelf.