QUOTE (lazyjones @ Jun 19 2008, 11:57 AM)

Don't let Lexus screw you. There is no "secret ohm" kit. All the speakers in the car are 8 ohms, except for the sub, which is 16 ohm. I just recently replaced all the speakers in my 2002 sc430, and the sound sounds 100% better than their "premium, Mark Levinson" system. I kept the factory head unit, as well as the factory ML amp to run the door speakers and rear speakers. Added an Alpine amp to power my new sub. You can find way better 8 ohm speakers than what is being used in the car. I did very little modifications to make the new speakers fit. In fact, the only modification I did, was changed the door woofers (which are 6 x 9) to 6.5" pioneer woofers. I had to use a little MDF to make the speakers fit. Was very simple. I changed out the rear speakers, and they fit fine. Changed out the sub, and it fit fine...(although, I wish I could have gone with a bigger sub. Only an 8" sub would replace the old one).
What kind of speakers did you exactly replace them with? I've yet to find an 8 Ohm car speaker. A 'home stereo' speaker isn't geared for this sort of application-- sonicly, or long term. I just broke down and purchased the ML 6X9 for the door as I blew the minimally rated 20W speaker. Do some searching on the web. I found one for $120. Anyway, my gripe is that playing music w/the top down is almost a joke if you're on the highway. Even replacing the sub becomes a big deal, as you're limited to where it can go-- plan on building a custom box in the back seat (as it's not very good for much else). You'll be better off placing it (them) here, than placing them in the trunk. It will sound better, and you will appreciate keeping the space available in the trunk should you need it. FYI, the existing sub cut out is far too shallow and small to replace it with anything-- even the shallow subs will not work, plus it will sound like garbage trying to play through the foam and leather seatback.
Unfortunately there's no real easy (or cheap) way to beef up the sound (or volume). ANY upgrades require a major overhaul, and you loose the asthetics of the car. Now, how about an aftermarket 8-Trak tape player to match the cassette deck?!!? LOL. Good Luck.