Help please -open circuit damage Melos 402 mono

I fried my speaker ,open circuit damage on my Melos 402 mono tube amp.It has four 6kg6a output, one 12au7 and 12ax7 pre.Two output tubes went bad.I thoroughly cleaned (Caig tube survival kit) all sockets and replaced all tubes.After turning on and warmed up enough to play, it performs like new for about 1/2 hour then it starts to crackle and distort in the 100 to 300 Hz range.When I shut it off,crackle crackle crackle.Any help on were to start looking would be greatly appreciated.
I cannot assist you with the repair of your amplifier. However, if you have not, or have decided not to repair the unit, I may be interested in purchasing them. Please advise andI will provide contact information. Thank you.