Why do my tube amps sound really good after the power is switched off and they start to die? If I switch one off and keep on playing, for a few seconds, at a low volume of course, I hear a really sweet overdrive that is better than when the amp is fully switched on. But then it fades away...
As the voltage falls after power-off, the headroom falls but so does the power output. What you're hearing has been called "brown" sound by some.
With cathode biased amps you can just feed them a little less voltage (the Variac method) but if you do that with a fixed bias amp you'll possibly burn out the output tubes and output transformer.
If you know how to re-bias your amp you can replace the rectifier tube (assuming it has one) with a Copper Cap rectifier from http://www.webervst.com with built-in voltage dropping. That will shift things towards the direction you want to go.
my guess is that it's cuz the voltage from anode to cathode is decreasing, and as you lower that voltage, you get more clipping. s'why people started using variacs to lower line voltage.