Look for an older thread on the Dynaudio system, an expensive Volvo-sponsored speaker upgrade.

The speakers in the back shelf are 8-inch paper cone 2-ways, that cost around $120 to replace. Some prople think that they have an annoying level of bass at around 60 Hz.

I installed Pioneer TS-A1780R 6 3/4 inch 4-way round speakers because Crutchfield recommended their fit. They are a drop-in install and give a pleasant, balanced sound. Don't worry about 4 ohm vs 8, they will work fine.

Installing 6x9s would require chopping the speaker mounts, depending on the particular speaker.

If you want hip-hop bass, you will need an amp and sub.