I had an Eclipse 3640 4 channel amplifier installed recently. It puts out 60W per channel to 4 channels, and can be setup to drive 2, 3, 4, or 5 speakers. It has high level as well as line level inputs, and high and low pass filters, as well as adjustable bass boost.

I am driving it using the line level signal from my stock Concert head unit. I attached female RCA plugs to the pinouts on the back of the deck to allow connection via regular RCA cables. It is mounted on the back of the left rear seat. Currently the amp is driving the stock speakers, and due to the excellent sound that it is now producing I am in no rush to replace them. The improvement over stock is truly remarkable. The bass is deeper, stronger, and more controlled (bass control is at 0 and the amp's bass boost is not in use). Mids and highs are smoother and cleaner, and clearer. At high volume the bass no longer gets unctrolled and muddy, and the mids and highs remain sweet rather than becoming strained as they did under stock power. I am extremely pleased with this product, it has markedly improved my A4's sound system, while not changing the stock appearance or functionality. It also offers me great flexability for future upgrades.