My fiance purchased her RX330 new in Feb 05 (w/o Nav and ML) and kept the head unit but upgraded to Boston Acoustics speakers powered by a 75x4 Alpine amplifier. In addition, she also included 2-10" subs powered by a 175x2 MTX amp for added bass. In all, it sounds pretty clean. The only reason she did not change the radio itself was 1. it was a lease, and 2. she woud loose the steering wheel controls.
BTW....if you are looking to have it professionally installed, steer clear of Tweeter. They raked up $3600 in damages to her truck (albeit it had 250 miles on it) - damage included scratches to the doors, wood gear selector and panels.
Hope this helps...