A four-channel amp or six-channel system would do, depending on whether you want more capacity for the tweeters. Few amps will be steady at 2 ohms, but mine will most likely be 3 ohms due to the parallel wiring. The resistance of an OHM is not always constant. Get a 50–100-watt RMS output power amp. The enhancement is nullified if the performance per speaker is less than 50 watts, but it should still be higher than the factory amp.