It could very well be tube type/family.I`ve had several 6550 tubes fail(friend same problem with some KT 88) early in their life. EL 34 were never a problem for me over many years.My current amp uses the 300b and these are the most reliable and best sounding of any tube I`ve used.\I wonder if the DHT tubes are inherently more rugged and durable.The DHT seem to last a lot longer also.
Regards,
Regards,