Yes, you MAY have destroyed the output tubes...

When replacing, make sure your MATCHED pairs go into V1 and V4 and the other into V2 and V3 respectively. Many don't realize the proper placement of matched pairs.

Open unit and if you find the tubes with the plates glowing red OR purple haze down between the elements, the tubes are gonzo.

Check the bias at connector CON1, Between pin 1 and 2 is a 1 ohm resistor in the cathode of the V1 and V2 side. Adjust the Bias1 for the number of milivolts matching the required bias current for two of the output tubes. Do the same for pins 2 and 3 of CON1 for the second side using the BIAS2 control. Fir your amp, you will want each of these set to 70millivolts.

It is possible (hopefully) that one of the smaller tubes burned out. These are much less expensive. EACH of these will have TWO points of light visible when they are operating. If you only see one, then 1/2 may be burned out. When turbes burn out, they cool down and sound will fade out.

There are other components that could cause your problem but those are not DIY unless you are adept at electronics.