How did you check the transistors after it laid down? Did you measure the hfe?
Sounds very much as if this amplifier is built in what they like to call the "competition" mode.
I call that design method "DFX", for DaveFatXforce.
They use the same design features, saturating the transistors at the highest possible power-supply voltage. Square law explains it. Increasing the power supply by half-again from 13.5 to 20 Volts will double the peak power the transistor can show.
More or less.
The math is not complex. Power is voltage times current. Increase the supply voltage by half and the circuit current should also increase to half-again more. Half-again times half-again is roughly two. Or a little more
I hear a subtext that this amplifier ran for some time with Toshiba transistors before blowing them out. The recent replacements are no doubt chinese parts that will match the conservative 100-Watt nominal peak power level at the rated power supply voltage.
Those same transistors most often do NOT last long in the 20-Volt hot-rod environment of a DFX amplifier.
Not long, like maybe a few keys of the mike, max.
What's needed for this kind of amplifier now that you can't get the super-tough Toshiba parts is a rating method.
I vote for a performance-rating number we'll call the "KBK".
As in "Keys Between Kabooms". How many times can you key the mike between sets of transistors.
Kinda like the unlimited alcohol/nitro drag-racing motors. Needs a total rebuild at the end of every 1/4-mile run. Valves are warped, electrodes burned off the spark plugs, etc.
Turning down the power supply to the rated 13 or so Volts is the only way to make the modern-day (post-modern?) transistors last very long at all.
73
