I am very happy with the Kenwood KACM1814 and KACM3004 in my 9-3s. Both are compact Class D 4 channel amps and fit where the factory amp goes... I reused all factory wiring. 30 minute install. I would definitely have to put in TONS more money and effort to net an improvement in sound, and I'm not sure it would be a practical improvement once done.