K 4040

I am owning 2 units of k 4040 valve amps. I had assembled one and was very happy all these years and planning to assemble the other one shortly. How ever last week while listening i cranked up the volume to the max( parasound zpre - pre amplifier). Suddenly BOTH the channel got distortion. Tubes are all showing normal glow. Wondering what might had happened ?? Any body can help in which area i should look??. I can repair the unit, if i get sime assistance. Thanks in advance- Raj nair.