I replaced my head unit with a junkyard unit twice (wore out the tape adapter mechanism twice haha) and added a factory 7-band (I think) equalizer. It makes things sound SO good compared to before. The model is TD-6141 or something-6141.

As for speakers, they really are not bad. Volvo did something nifty and basically made a 4-way speaker setup in every 700 I've ever seen. The rear woofers have lower tones and larger dome tweeters (meant to be mids I think), the front woofers have higher tones and small plastic-film tweeters. This is great because the downfall of most round door-mount speakers is having too much frequency/pitch spread on one woofer. Splitting it up really improves the stereo system. The stereo amp and speakers are actually quite nice if paired with something that can give good quality input to it - like an aftermarket unit, or the stock unit with the 7-band EQ.

If you're really going to do up the car, I'd say keep the stock speakers, get a new head unit, add an amp and drive 6 speakers including a pair of 6x9 (or 5x7?) speakers in the rear deck. There's space for the speakers and to mount an amp in the back. That's getting pretty deep into it though.

Cheers!

PS: It's also worth mentioning that if the cables to the back of the stereo unit are loose, that can cause this lack of sound. There are 2 fuses in the back of the stereo too that can blow, but I've never seen them blow. The round DIN shell cable runs to the amp, and there should be a beige rubber cap with 4 pins stuck into another port on the back of the stereo. That's where the 7-band EQ plugs in. If that plug is missing, you'll get no sound. Same with the DIN shell (like an older keyboard plug)