Okay, explain this to me like I'm a 5 year old: What are the sonic differences between power tube overdrive and preamp tube OD? Is one preferable to the other? I know that with a gain/volume control, you can play with OD at lower volumes, but is there something I'm missing here? Why wouldn't all amps be set up this way? It seems to me that there would be more control over your sound this way. Is there some advantage to having just a volume control and relying on power tube OD?