im not talking shit about your amp and i understand people are sensitive when they hear their amp isnt doing what they think. im just telling you the truth and making sure you are set up for the extra power so you dont break something else. yes, im actually helping. now its not called "watts law" its ohm's law. resistance definitely plays a part and so does efficiency. something your math didnt account for.
14.4 x 60 = 864
864 x .75 (efficiency) = 648 (hey look at that, i was pretty darn close)
then account for your voltage drop and im probably rite on the money.
lastly, doubling the resistance should cut the power in half. this does seem to varry based on the amp's design but lets say, best case,(and your amp wont be this good) it drops %30 leaving you with 453w at perfect voltage. then we factor in impedance rise and holy fuck lol. you dont even want to know what that brings you to.
now there is good news here. if you were happy with the output of this kenwood amp, you dont need to look for a 1000w replacement. just get an amp that truely puts out 500w at 2ohm and you will be set. that will really help you save money and get a better quality amp.