Amazon.com: Infinity Kappa Four - Four Channel Class D Car Audio Amplifier : Electronics
Diy) Amp needed for Infinity Renaissance 90 | diyAudio
Infinity Kappa Four AM Amplifer 2/3/4 Channel Class D Car Audio 100W X 4 Rms 50667373146 | eBay
Archimago's Musings: MEASUREMENTS: Melody Onix SP3 Mk. II (~2006) Class AB Tube Integrated Amplifier. (And it isn't "diminishing returns" on sound quality, often there are simply "no returns".)