Review on Infinity Reference 551a 550W x 1 Car Amplifier: Unleash Power and Performance on the Road by Tony Korek

Excellent amp. It and the JBL Club 5501 use the same Harman Kardon amplifier circuit. I'm sure there is another high end model that uses the same board as both amps have features not listed anywhere in their literature which both JBL and Infinity dispute. In particular, the Reference 551a and Club 5501 are able to receive an audio signal for switching on and off instead of using a dedicated remote line. They can also deliver slightly more power than rated, which is fairly rare. I don't think you can get a better amp for $140. Certainly not new.