i know for a fact that they recorded the guitar parts at LOW volumes, which is unusual for studios to do. They also used nice compressors (like UREI or whatever ).
THis album can be listened to at very low volumes and still be "FELT". When you turn it up, it has the same dynamic impact.
THis is done greatly through compression. I'm not saying buying a compressor is the secret to the sound, but if you listen to weezer live, their guitars sound very different from that album.
The album: Low volumes, quality compression and good mixing.