making power from kt120 tubes

what is the main reason a amp that is rated at 60 wpc with kt88,6550's will only get about 5-10 watts more a channel with kt120's or the new kt150's. since these tubes are rated for about 60 watts dissipation each. a 60 wpc amp with two tubes per channel you would think you would get 100-120wpc. but an amp designed for 60 wpc will only see a little more power. upping the plate volts from 500 to 500-600 would make more power?? most tubes you do that with everybody says the tubes would work harder and burn out faster. but with 120's,everybody says they are not working hard at all if you set to factory bias settings. is the screen voltage the limiting factor. if so cant the amp be adjusted to raise that and get more power out of the tube. just wondering here. everybody is buying these new tubes because their amp can handle them but your not getting the power potential out of them. thanks for your input